diff --git a/papers/commands/tags_cmd.py b/papers/commands/tags_cmd.py
index c56384d..7d8eab7 100644
--- a/papers/commands/tags_cmd.py
+++ b/papers/commands/tags_cmd.py
@@ -1,13 +1,59 @@
-from ..repo import Repository
+"""
+This command is all about tags.
+The different use cases are :
+1. > papers tag
+    Returns the list of all tags
+2. > papers tag citekey
+    Return the list of tags of the given citekey
+3. > papers tag citekey math
+    Add 'math' to the list of tags of the given citekey
+4. > papers tag citekey :math
+    Remove 'math' for the list of tags of the given citekey
+5. > papers tag citekey math,romance,:war
+    Add 'math' and 'romance' tags to the given citekey, and remove the 'war' tag
+6. > papers tag math
+    If 'math' is not a citekey, then display all papers with the tag 'math'
+"""
 
+from ..repo import Repository, InvalidReference
+from . import helpers
 
 def parser(subparsers, config):
-    parser = subparsers.add_parser('tags', help="list existing tags")
+    parser = subparsers.add_parser('tags', help="add, remove and show tags")
+    parser.add_argument('referenceOrTag', nargs='?', default = None,
+                        help='reference to the paper (citekey or number), or '
+                             'tag.')
+    parser.add_argument('tags', nargs='?', default = None,
+                        help='If the previous argument was a reference, then '
+                             'then a list of tags separated by commas.')
+    # TODO find a way to display clear help for multiple command semantics,
+    #      indistinguisable for argparse. (fabien, 201306)
     return parser
 
-
-def command(config, ui):
-    """List existing tags"""
+def command(config, ui, referenceOrTag, tags):
+    """Add, remove and show tags"""
     rp = Repository.from_directory(config)
-    for tag in rp.get_tags():
-        ui.print_(tag)
+
+    if referenceOrTag is None:
+        for tag in rp.get_tags():
+            ui.print_(tag)
+    else:
+        try:
+            citekey = rp.citekey_from_ref(referenceOrTag)
+            p = rp.get_paper(citekey)
+            if tags is None:
+                ui.print_(' '.join(p.tags))
+            else:
+                tags = tags.split(',')
+                for tag in tags:
+                    if tag[0] == ':':
+                        p.remove_tag(tag[1:])
+                    else:
+                        p.add_tag(tag)
+                rp.save_paper(p)
+        except InvalidReference:
+            tag = referenceOrTag
+            papers_list = [(p, n) for n, p in enumerate(rp.all_papers())
+                           if tag in p.tags]
+            ui.print_('\n'.join(helpers.paper_oneliner(p, n)
+                                for p, n in papers_list))
\ No newline at end of file
diff --git a/papers/paper.py b/papers/paper.py
index 55d5705..037b5a6 100644
--- a/papers/paper.py
+++ b/papers/paper.py
@@ -69,6 +69,7 @@ def get_safe_metadata(meta):
     base_meta = Paper.create_meta()
     if meta is not None:
         base_meta.update(meta)
+    base_meta['tags'] = set(base_meta['tags'])
     return base_meta
 
 
@@ -266,11 +267,8 @@ class Paper(object):
         self.tags.add(tag)
 
     def remove_tag(self, tag):
-        """Remove a tag from a paper. Fails silently."""
-        try:
-            self.tags.pop(tag)
-        except KeyError:
-            pass
+        """Remove a tag from a paper if present."""
+        self.tags.discard(tag)
 
 
 class PaperInRepo(Paper): # TODO document why this class exists (fabien, 2013/06)