diff --git a/chatmastermind/tags.py b/chatmastermind/tags.py
new file mode 100644
index 0000000..6360636
--- /dev/null
+++ b/chatmastermind/tags.py
@@ -0,0 +1,128 @@
+"""
+Module implementing tag related functions and classes.
+"""
+from typing import Type, TypeVar
+
+TagInst = TypeVar('TagInst', bound='Tag')
+TagLineInst = TypeVar('TagLineInst', bound='TagLine')
+
+
+class TagError(Exception):
+    pass
+
+
+class Tag(str):
+    """
+    A single tag. A string that can contain anything but the default separator (' ').
+    """
+    # default separator
+    default_separator = ' '
+    # alternative separators (e. g. for backwards compatibility)
+    alternative_separators = [',']
+
+    def __new__(cls: Type[TagInst], string: str) -> TagInst:
+        """
+        Make sure the tag string does not contain the default separator.
+        """
+        if cls.default_separator in string:
+            raise TypeError(f"Tag '{string}' contains the separator char '{cls.default_separator}'")
+        instance = super().__new__(cls, string)
+        return instance
+
+
+class TagLine(str):
+    """
+    A line of tags. It starts with a prefix ('TAGS:'), followed by a list of tags,
+    separated by the defaut separator (' '). Any operations on a TagLine will sort
+    the tags.
+    """
+    # the prefix
+    prefix = 'TAGS:'
+
+    def __new__(cls: Type[TagLineInst], string: str) -> TagLineInst:
+        """
+        Make sure the tagline string starts with the prefix.
+        """
+        if not string.startswith(cls.prefix):
+            raise TagError(f"TagLine '{string}' is missing prefix '{cls.prefix}'")
+        instance = super().__new__(cls, string)
+        return instance
+
+    @classmethod
+    def from_set(cls: Type[TagLineInst], tags: set[Tag]) -> TagLineInst:
+        """
+        Create a new TagLine from a set of tags.
+        """
+        return cls(' '.join([TagLine.prefix] + sorted([t for t in tags])))
+
+    def tags(self) -> set[Tag]:
+        """
+        Returns all tags contained in this line as a set.
+        """
+        tagstr = self[len(self.prefix):].strip()
+        separator = Tag.default_separator
+        # look for alternative separators and use the first one found
+        # -> we don't support different separators in the same TagLine
+        for s in Tag.alternative_separators:
+            if s in tagstr:
+                separator = s
+                break
+        return set(sorted([Tag(t.strip()) for t in tagstr.split(separator)]))
+
+    def merge(self, taglines: set['TagLine']) -> 'TagLine':
+        """
+        Merges the tags of all given taglines into the current one
+        and returns a new TagLine.
+        """
+        merged_tags = self.tags()
+        for tl in taglines:
+            merged_tags |= tl.tags()
+        return self.from_set(set(sorted(merged_tags)))
+
+    def delete_tags(self, tags: set[Tag]) -> 'TagLine':
+        """
+        Deletes the given tags and returns a new TagLine.
+        """
+        return self.from_set(self.tags().difference(tags))
+
+    def add_tags(self, tags: set[Tag]) -> 'TagLine':
+        """
+        Adds the given tags and returns a new TagLine.
+        """
+        return self.from_set(set(sorted(self.tags() | tags)))
+
+    def rename_tags(self, tags: set[tuple[Tag, Tag]]) -> 'TagLine':
+        """
+        Renames the given tags and returns a new TagLine. The first
+        tuple element is the old name, the second one is the new name.
+        """
+        new_tags = self.tags()
+        for t in tags:
+            if t[0] in new_tags:
+                new_tags.remove(t[0])
+                new_tags.add(t[1])
+        return self.from_set(set(sorted(new_tags)))
+
+    def match_tags(self, tags_or: set[Tag], tags_and: set[Tag], tags_not: set[Tag]) -> bool:
+        """
+        Checks if the current TagLine matches the given tag requirements:
+        - 'tags_or' : matches if this TagLine contains ANY of those tags
+        - 'tags_and': matches if this TagLine contains ALL of those tags
+        - 'tags_not': matches if this TagLine contains NONE of those tags
+
+        Note that it's sufficient if the TagLine matches one of 'tags_or' or 'tags_and',
+        i. e. you can select a TagLine if it either contains one of the tags in 'tags_or'
+        or all of the tags in 'tags_and' but it must never contain any of the tags in
+        'tags_not'. If a given tag set is 'None', it matches all tags.
+        """
+        tag_set = self.tags()
+        required_tags_present = False
+        excluded_tags_missing = False
+        if ((tags_or is None and tags_and is None)
+           or (tags_or and any(tag in tag_set for tag in tags_or))   # noqa: W503
+           or (tags_and and all(tag in tag_set for tag in tags_and))):  # noqa: W503
+            required_tags_present = True
+        if ((tags_not is None)
+           or (not any(tag in tag_set for tag in tags_not))):  # noqa: W503
+            excluded_tags_missing = True
+        return required_tags_present and excluded_tags_missing