diff --git a/NEWS b/NEWS
index df5852b90..65586e55b 100644
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,11 @@
 1.0b10
 ------
+* A new "beet modify" command enables manual, command-line-based
+  modification of music metadata. Pass it a query along with
+  field=value that specify the changes you want to make.
+* Handle exceptions thrown when running Mutagen.
 * Fix a missing __future__ import in embedart on Python 2.5.
+* Fix ID3 and MPEG-4 tag names for the album-artist field.
 * Fix Unicode encoding of album artist, album type, and label.
 
 1.0b9
diff --git a/beets/library.py b/beets/library.py
index 44490aba6..7986cd0e4 100644
--- a/beets/library.py
+++ b/beets/library.py
@@ -250,8 +250,13 @@ class Item(object):
                 shutil.move(syspath(self.path), syspath(dest))
             
         # Either copying or moving succeeded, so update the stored path.
+        old_path = self.path
         self.path = dest
 
+        # Prune vacated directory.
+        if not copy:
+            util.prune_dirs(os.path.dirname(old_path), library.directory)
+
 
 # Library queries.
 
@@ -729,8 +734,11 @@ class Library(BaseLibrary):
                        art_filename='cover',
                        item_fields=ITEM_FIELDS,
                        album_fields=ALBUM_FIELDS):
-        self.path = bytestring_path(path)
-        self.directory = bytestring_path(directory)
+        if path == ':memory:':
+            self.path = path
+        else:
+            self.path = bytestring_path(normpath(path))
+        self.directory = bytestring_path(normpath(directory))
         if path_formats is None:
             path_formats = {'default': '$artist/$album/$track $title'}
         elif isinstance(path_formats, basestring):
@@ -1248,6 +1256,9 @@ class Album(BaseAlbum):
                 else:
                     shutil.move(syspath(old_art), syspath(new_art))
                 self.artpath = new_art
+            if not copy: # Prune old path.
+                util.prune_dirs(os.path.dirname(old_art),
+                                self._library.directory)
 
         # Store new item paths. We do this at the end to avoid
         # locking the database for too long while files are copied.
diff --git a/beets/mediafile.py b/beets/mediafile.py
index b31b638a1..12281f1f5 100644
--- a/beets/mediafile.py
+++ b/beets/mediafile.py
@@ -39,11 +39,17 @@ import re
 import base64
 import imghdr
 import os
+import logging
+import traceback
 from beets.util.enumeration import enum
 
 __all__ = ['UnreadableFileError', 'FileTypeError', 'MediaFile']
 
 
+# Logger.
+log = logging.getLogger('beets')
+
+
 # Exceptions.
 
 # Raised for any file MediaFile can't read.
@@ -382,6 +388,11 @@ class MediaField(object):
         
         if style.packing:
             out = Packed(out, style.packing)[style.pack_pos]
+
+        # MPEG-4 freeform frames are (should be?) encoded as UTF-8.
+        if obj.type == 'mp4' and style.key.startswith('----:') and \
+                isinstance(out, str):
+            out = out.decode('utf8')
         
         return _safe_cast(self.out_type, out)
     
@@ -410,8 +421,8 @@ class MediaField(object):
                         out = u''
                     # We trust that packed values are handled above.
         
-                # convert to correct storage type (irrelevant for
-                # packed values)
+                # Convert to correct storage type (irrelevant for
+                # packed values).
                 if style.as_type == unicode:
                     if out is None:
                         out = u''
@@ -429,7 +440,13 @@ class MediaField(object):
                 elif style.as_type in (bool, str):
                     out = style.as_type(out)
         
-            # store the data
+            # MPEG-4 "freeform" (----) frames must be encoded as UTF-8
+            # byte strings.
+            if obj.type == 'mp4' and style.key.startswith('----:') and \
+                    isinstance(out, unicode):
+                out = out.encode('utf8')
+
+            # Store the data.
             self._storedata(obj, out, style)
 
 class CompositeDateField(object):
@@ -619,9 +636,14 @@ class MediaFile(object):
         try:
             self.mgfile = mutagen.File(path)
         except unreadable_exc:
+            log.warn('header parsing failed')
             raise UnreadableFileError('Mutagen could not read file')
         except IOError:
             raise UnreadableFileError('could not read file')
+        except:
+            # Hide bugs in Mutagen.
+            log.error('uncaught Mutagen exception:\n' + traceback.format_exc())
+            raise UnreadableFileError('Mutagen raised an exception')
 
         if self.mgfile is None: # Mutagen couldn't guess the type
             raise FileTypeError('file type unsupported by Mutagen')
@@ -788,9 +810,8 @@ class MediaFile(object):
                 etc = StorageStyle('compilation')
             )
     albumartist = MediaField(
-                mp3 = StorageStyle('TXXX', id3_desc=u'Album Artist'),
-                mp4 = StorageStyle(
-                    '----:com.apple.iTunes:Album Artist'),
+                mp3 = StorageStyle('TPE2'),
+                mp4 = StorageStyle('aART'),
                 etc = [StorageStyle('album artist'),
                        StorageStyle('albumartist')]
             )
diff --git a/beets/ui/commands.py b/beets/ui/commands.py
index f2a3acfd6..ee859da73 100755
--- a/beets/ui/commands.py
+++ b/beets/ui/commands.py
@@ -21,6 +21,7 @@ import sys
 import os
 import time
 import copy
+import itertools
 
 from beets import ui
 from beets.ui import print_, decargs
@@ -28,8 +29,9 @@ from beets import autotag
 import beets.autotag.art
 from beets import plugins
 from beets import importer
-from beets.util import syspath, normpath
 from beets.library import ITEM_MODIFIED, ITEM_DELETED
+from beets.util import syspath, normpath, ancestry
+from beets import library
 
 # Global logger.
 log = logging.getLogger('beets')
@@ -38,6 +40,32 @@ log = logging.getLogger('beets')
 # objects that can be fed to a SubcommandsOptionParser.
 default_commands = []
 
+# Utility.
+def _do_query(lib, query, album, also_items=True):
+    """For commands that operate on matched items, performs a query
+    and returns a list of matching items and a list of matching
+    albums. (The latter is only nonempty when album is True.) Raises
+    a UserError if no items match. also_items controls whether, when
+    fetching albums, the associated items should be fetched also.
+    """
+    if album:
+        albums = list(lib.albums(query))
+        items = []
+        if also_items:
+            for al in albums:
+                items += al.items()
+
+    else:
+        albums = []
+        items = list(lib.items(query))
+
+    if album and not albums:
+        raise ui.UserError('No matching albums found.')
+    elif not album and not items:
+        raise ui.UserError('No matching items found.')
+    
+    return items, albums
+
 
 # import: Autotagger and importer.
 
@@ -701,17 +729,7 @@ def remove_items(lib, query, album, delete=False):
     remove whole albums. If delete, also remove files from disk.
     """
     # Get the matching items.
-    if album:
-        albums = list(lib.albums(query))
-        items = []
-        for al in albums:
-            items += al.items()
-    else:
-        items = list(lib.items(query))
-
-    if not items:
-        print_('No matching items found.')
-        return
+    items, albums = _do_query(lib, query, album)
 
     # Show all the items.
     for item in items:
@@ -807,3 +825,97 @@ version_cmd = ui.Subcommand('version',
     help='output version information')
 version_cmd.func = show_version
 default_commands.append(version_cmd)
+
+
+# modify: Declaratively change metadata.
+
+def modify_items(lib, mods, query, write, move, album, color, confirm):
+    """Modifies matching items according to key=value assignments."""
+    # Parse key=value specifications into a dictionary.
+    allowed_keys = library.ALBUM_KEYS if album else library.ITEM_KEYS_WRITABLE
+    fsets = {}
+    for mod in mods:
+        key, value = mod.split('=', 1)
+        if key not in allowed_keys:
+            raise ui.UserError('"%s" is not a valid field' % key)
+        fsets[key] = value
+
+    # Get the items to modify.
+    items, albums = _do_query(lib, query, album, False)
+    objs = albums if album else items
+
+    # Preview change.
+    print_('Modifying %i %ss.' % (len(objs), 'album' if album else 'item'))
+    for obj in objs:
+        # Identify the changed object.
+        if album:
+            print_(u'* %s - %s' % (obj.albumartist, obj.album))
+        else:
+            print_(u'* %s - %s' % (obj.artist, obj.title))
+
+        # Show each change.
+        for field, value in fsets.iteritems():
+            curval = getattr(obj, field)
+            if curval != value:
+                if color:
+                    curval, value = ui.colordiff(curval, value)
+                print_(u'  %s: %s -> %s' % (field, curval, value))
+
+    # Confirm.
+    if confirm:
+        extra = ' and write tags' if write else ''
+        if not ui.input_yn('Really modify%s (Y/n)?' % extra):
+            return
+
+    # Apply changes to database.
+    for obj in objs:
+        for field, value in fsets.iteritems():
+            setattr(obj, field, value)
+
+        if move:
+            cur_path = obj.item_dir() if album else obj.path
+            if lib.directory in ancestry(cur_path): # In library?
+                log.debug('moving object %s' % cur_path)
+                if album:
+                    obj.move()
+                else:
+                    obj.move(lib)
+
+        # When modifying items, we have to store them to the database.
+        if not album:
+            lib.store(obj)
+    lib.save()
+
+    # Apply tags if requested.
+    if write:
+        if album:
+            items = itertools.chain(*(a.items() for a in albums))
+        for item in items:
+            item.write()
+
+modify_cmd = ui.Subcommand('modify',
+    help='change metadata fields', aliases=('mod',))
+modify_cmd.parser.add_option('-M', '--nomove', action='store_false',
+    default=True, dest='move', help="don't move files in library")
+modify_cmd.parser.add_option('-w', '--write', action='store_true',
+    default=None, help="write new metadata to files' tags (default)")
+modify_cmd.parser.add_option('-W', '--nowrite', action='store_false',
+    dest='write', help="don't write metadata (opposite of -w)")
+modify_cmd.parser.add_option('-a', '--album', action='store_true',
+    help='modify whole albums instead of tracks')
+modify_cmd.parser.add_option('-y', '--yes', action='store_true',
+    help='skip confirmation')
+def modify_func(lib, config, opts, args):
+    args = decargs(args)
+    mods = [a for a in args if '=' in a]
+    query = [a for a in args if '=' not in a]
+    if not mods:
+        raise ui.UserError('no modifications specified')
+    write = opts.write if opts.write is not None else \
+        ui.config_val(config, 'beets', 'import_write',
+            DEFAULT_IMPORT_WRITE, bool)
+    color = ui.config_val(config, 'beets', 'color', DEFAULT_COLOR, bool)
+    modify_items(lib, mods, query, write, opts.move, opts.album, color,
+                 not opts.yes)
+modify_cmd.func = modify_func
+default_commands.append(modify_cmd)
diff --git a/beets/util/__init__.py b/beets/util/__init__.py
index 041981106..239294a74 100644
--- a/beets/util/__init__.py
+++ b/beets/util/__init__.py
@@ -180,7 +180,7 @@ CHAR_REPLACE = [
     (re.compile(r'[\\/\?]|^\.'), '_'),
     (re.compile(r':'), '-'),
 ]
-CHAR_REPLACE_WINDOWS = re.compile('["\*<>\|]|^\.|\.$| +$'), '_'
+CHAR_REPLACE_WINDOWS = re.compile(r'["\*<>\|]|^\.|\.$| +$'), '_'
 def sanitize_path(path, pathmod=None):
     """Takes a path and makes sure that it is legal. Returns a new path.
     Only works with fragments; won't work reliably on Windows when a
diff --git a/test/test_files.py b/test/test_files.py
index b534721aa..b5a7b42c6 100644
--- a/test/test_files.py
+++ b/test/test_files.py
@@ -26,7 +26,7 @@ from _common import item, touch
 import beets.library
 from beets import util
 
-class MoveTest(unittest.TestCase):
+class MoveTest(unittest.TestCase, _common.ExtraAsserts):
     def setUp(self):
         # make a temporary file
         self.path = join(_common.RSRC, 'temp.mp3')
@@ -54,19 +54,29 @@ class MoveTest(unittest.TestCase):
     
     def test_move_arrives(self):
         self.i.move(self.lib)
-        self.assertTrue(os.path.exists(self.dest))
+        self.assertExists(self.dest)
     
     def test_move_departs(self):
         self.i.move(self.lib)
-        self.assertTrue(not os.path.exists(self.path))
+        self.assertNotExists(self.path)
+
+    def test_move_in_lib_prunes_empty_dir(self):
+        self.i.move(self.lib)
+        old_path = self.i.path
+        self.assertExists(old_path)
+
+        self.i.artist = 'newArtist'
+        self.i.move(self.lib)
+        self.assertNotExists(old_path)
+        self.assertNotExists(os.path.dirname(old_path))
     
     def test_copy_arrives(self):
         self.i.move(self.lib, copy=True)
-        self.assertTrue(os.path.exists(self.dest))
+        self.assertExists(self.dest)
     
     def test_copy_does_not_depart(self):
         self.i.move(self.lib, copy=True)
-        self.assertTrue(os.path.exists(self.path))
+        self.assertExists(self.path)
     
     def test_move_changes_path(self):
         self.i.move(self.lib)
@@ -257,7 +267,7 @@ class RemoveTest(unittest.TestCase):
         if os.path.exists(self.libdir):
             shutil.rmtree(self.libdir)
 
-    def test_removing_last_item_removes_empty_dir(self):
+    def test_removing_last_item_prunes_empty_dir(self):
         parent = os.path.dirname(self.i.path)
         self.assertTrue(os.path.exists(parent))
         self.lib.remove(self.i, True)
diff --git a/test/test_mediafile.py b/test/test_mediafile.py
index 97cf8bc20..5fa7cdd62 100644
--- a/test/test_mediafile.py
+++ b/test/test_mediafile.py
@@ -17,6 +17,7 @@
 
 import unittest
 import os
+import shutil
 
 import _common
 import beets.mediafile
@@ -149,6 +150,23 @@ class SideEffectsTest(unittest.TestCase):
         new_mtime = os.stat(self.empty).st_mtime
         self.assertEqual(old_mtime, new_mtime)
 
+class EncodingTest(unittest.TestCase):
+    def setUp(self):
+        src = os.path.join(_common.RSRC, 'full.m4a')
+        self.path = os.path.join(_common.RSRC, 'test.m4a')
+        shutil.copy(src, self.path)
+
+        self.mf = beets.mediafile.MediaFile(self.path)
+
+    def tearDown(self):
+        os.remove(self.path)
+
+    def test_unicode_label_in_m4a(self):
+        self.mf.label = u'foo\xe8bar'
+        self.mf.save()
+        new_mf = beets.mediafile.MediaFile(self.path)
+        self.assertEqual(new_mf.label, u'foo\xe8bar')
+
 def suite():
     return unittest.TestLoader().loadTestsFromName(__name__)
 
diff --git a/test/test_ui.py b/test/test_ui.py
index 99de272a6..721f5dd9f 100644
--- a/test/test_ui.py
+++ b/test/test_ui.py
@@ -119,6 +119,85 @@ class RemoveTest(unittest.TestCase):
         self.assertEqual(len(list(items)), 0)
         self.assertFalse(os.path.exists(self.i.path))
 
+class ModifyTest(unittest.TestCase):
+    def setUp(self):
+        self.io = _common.DummyIO()
+        self.io.install()
+
+        self.libdir = os.path.join(_common.RSRC, 'testlibdir')
+        os.mkdir(self.libdir)
+
+        # Copy a file into the library.
+        self.lib = library.Library(':memory:', self.libdir)
+        self.i = library.Item.from_path(os.path.join(_common.RSRC, 'full.mp3'))
+        self.lib.add(self.i, True)
+        self.album = self.lib.add_album([self.i])
+
+    def tearDown(self):
+        self.io.restore()
+        shutil.rmtree(self.libdir)
+
+    def _modify(self, mods, query=(), write=False, move=False, album=False):
+        self.io.addinput('y')
+        commands.modify_items(self.lib, mods, query,
+                              write, move, album, True, True)
+
+    def test_modify_item_dbdata(self):
+        self._modify(["title=newTitle"])
+        item = self.lib.items().next()
+        self.assertEqual(item.title, 'newTitle')
+
+    def test_modify_album_dbdata(self):
+        self._modify(["album=newAlbum"], album=True)
+        album = self.lib.albums()[0]
+        self.assertEqual(album.album, 'newAlbum')
+
+    def test_modify_item_tag_unmodified(self):
+        self._modify(["title=newTitle"], write=False)
+        item = self.lib.items().next()
+        item.read()
+        self.assertEqual(item.title, 'full')
+
+    def test_modify_album_tag_unmodified(self):
+        self._modify(["album=newAlbum"], write=False, album=True)
+        item = self.lib.items().next()
+        item.read()
+        self.assertEqual(item.album, 'the album')
+
+    def test_modify_item_tag(self):
+        self._modify(["title=newTitle"], write=True)
+        item = self.lib.items().next()
+        item.read()
+        self.assertEqual(item.title, 'newTitle')
+
+    def test_modify_album_tag(self):
+        self._modify(["album=newAlbum"], write=True, album=True)
+        item = self.lib.items().next()
+        item.read()
+        self.assertEqual(item.album, 'newAlbum')
+
+    def test_item_move(self):
+        self._modify(["title=newTitle"], move=True)
+        item = self.lib.items().next()
+        self.assertTrue('newTitle' in item.path)
+
+    def test_album_move(self):
+        self._modify(["album=newAlbum"], move=True, album=True)
+        item = self.lib.items().next()
+        item.read()
+        self.assertTrue('newAlbum' in item.path)
+
+    def test_item_not_move(self):
+        self._modify(["title=newTitle"], move=False)
+        item = self.lib.items().next()
+        self.assertFalse('newTitle' in item.path)
+
+    def test_album_not_move(self):
+        self._modify(["album=newAlbum"], move=False, album=True)
+        item = self.lib.items().next()
+        item.read()
+        self.assertFalse('newAlbum' in item.path)
+
 class PrintTest(unittest.TestCase):
     def setUp(self):
         self.io = _common.DummyIO()