diff --git a/src/lib.rs b/src/lib.rs
index affc408..5c7b632 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,4 +5,9 @@ pub mod regex;
 mod python_bindings;
 
 mod primitives;
-pub use primitives::{State, TokenId, TransitionKey};
+pub use primitives::{State, Token, TokenId, TransitionKey};
+
+mod vocabulary;
+pub use vocabulary::Vocabulary;
+
+pub(crate) use {std::collections::HashMap, std::ops::Deref};
diff --git a/src/primitives.rs b/src/primitives.rs
index bbc7770..e12bf03 100644
--- a/src/primitives.rs
+++ b/src/primitives.rs
@@ -1,6 +1,9 @@
 /// Interegular transition key.
 pub type TransitionKey = u32;
 
+/// Token content.
+pub type Token = String;
+
 /// Token identifier.
 pub type TokenId = u32;
 
diff --git a/src/python_bindings/mod.rs b/src/python_bindings/mod.rs
index 251300f..34cbf4f 100644
--- a/src/python_bindings/mod.rs
+++ b/src/python_bindings/mod.rs
@@ -97,6 +97,7 @@ pub fn state_scan_tokens_py(
     vocabulary_transition_keys: Vec<Vec<TransitionKey>>,
     start_state: State,
 ) -> PyResult<HashSet<(TokenId, State)>> {
+    let vocabulary = Vocabulary::from_iter(vocabulary);
     Ok(state_scan_tokens(
         &fsm_transitions,
         fsm_initial,
@@ -131,6 +132,7 @@ pub fn get_vocabulary_transition_keys_py(
     vocabulary: Vec<(String, Vec<TokenId>)>,
     frozen_tokens: HashSet<String>,
 ) -> PyResult<Vec<Vec<TransitionKey>>> {
+    let vocabulary = Vocabulary::from_iter(vocabulary);
     Ok(get_vocabulary_transition_keys(
         &alphabet_symbol_mapping,
         alphabet_anything_value,
@@ -147,6 +149,8 @@ pub fn create_fsm_index_end_to_end_py<'py>(
     vocabulary: Vec<(String, Vec<TokenId>)>,
     frozen_tokens: HashSet<String>,
 ) -> PyResult<Bound<'py, PyDict>> {
+    let vocabulary = Vocabulary::from_iter(vocabulary);
+
     let states_to_token_subsets = PyDict::new_bound(py);
     let mut seen: HashSet<State> = HashSet::new();
     let mut next_states: HashSet<State> = HashSet::from_iter(vec![fsm_info.initial]);
diff --git a/src/regex.rs b/src/regex.rs
index 314ff0c..c0eda76 100644
--- a/src/regex.rs
+++ b/src/regex.rs
@@ -42,7 +42,7 @@ pub fn state_scan_tokens(
     fsm_transitions: &HashMap<(State, TransitionKey), State>,
     fsm_initial: State,
     fsm_finals: &HashSet<State>,
-    vocabulary: &[(String, Vec<TokenId>)],
+    vocabulary: &Vocabulary,
     vocabulary_transition_keys: &[Vec<TransitionKey>],
     start_state: State,
 ) -> HashSet<(TokenId, State)> {
@@ -110,7 +110,7 @@ pub fn get_token_transition_keys(
 pub fn get_vocabulary_transition_keys(
     alphabet_symbol_mapping: &HashMap<String, TransitionKey>,
     alphabet_anything_value: TransitionKey,
-    vocabulary: &[(String, Vec<TokenId>)],
+    vocabulary: &Vocabulary,
     frozen_tokens: &HashSet<String>,
 ) -> Vec<Vec<TransitionKey>> {
     let mut vocab_transition_keys: Vec<Vec<TransitionKey>> = Vec::new();
diff --git a/src/vocabulary.rs b/src/vocabulary.rs
new file mode 100644
index 0000000..f926339
--- /dev/null
+++ b/src/vocabulary.rs
@@ -0,0 +1,101 @@
+use crate::*;
+
+/// Vocabulary of an LLM.
+///
+/// ## Examples
+///
+/// ```rust
+/// # use outlines_core::*;
+/// #
+/// let vocabulary = Vocabulary::new()
+///     .insert(0, "blah")
+///     .insert(1, "1a")
+///     .insert(2, "2")
+///     .insert(3, "0");
+/// ```
+#[derive(Clone, Debug, Default)]
+pub struct Vocabulary(HashMap<Token, Vec<TokenId>>);
+
+impl Vocabulary {
+    /// Creates an empty vocabulary.
+    pub fn new() -> Vocabulary {
+        Vocabulary::default()
+    }
+}
+
+impl Vocabulary {
+    /// Inserts a token to the vocabulary with the specified identifier.
+    pub fn insert(mut self, id: TokenId, token: impl Into<Token>) -> Vocabulary {
+        let token = token.into();
+        self.0.entry(token).or_default().push(id);
+        self
+    }
+
+    /// Extends the vocabulary with tokens and their identifiers.
+    pub fn extend<T: Into<Token>, I: IntoIterator<Item = TokenId>>(
+        mut self,
+        tokens_and_ids: impl IntoIterator<Item = (T, I)>,
+    ) -> Vocabulary {
+        for (token, ids) in tokens_and_ids.into_iter() {
+            let token = token.into();
+            for id in ids {
+                self = self.insert(id, token.clone());
+            }
+        }
+        self
+    }
+}
+
+impl Deref for Vocabulary {
+    type Target = HashMap<Token, Vec<TokenId>>;
+
+    fn deref(&self) -> &HashMap<Token, Vec<TokenId>> {
+        &self.0
+    }
+}
+
+impl<T, I> FromIterator<(T, I)> for Vocabulary
+where
+    T: Into<Token>,
+    I: IntoIterator<Item = TokenId>,
+{
+    fn from_iter<A: IntoIterator<Item = (T, I)>>(tokens_and_ids: A) -> Self {
+        Vocabulary::new().extend(tokens_and_ids)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::*;
+
+    #[test]
+    fn insert() {
+        let vocabulary = Vocabulary::new()
+            .insert(0, "blah")
+            .insert(1, "1a")
+            .insert(2, "2")
+            .insert(3, "0");
+
+        assert_eq!(vocabulary.len(), 4);
+        assert_eq!(vocabulary["blah"], &[0]);
+        assert_eq!(vocabulary["1a"], &[1]);
+        assert_eq!(vocabulary["2"], &[2]);
+        assert_eq!(vocabulary["0"], &[3]);
+    }
+
+    #[test]
+    fn extend() {
+        let vocabulary = Vocabulary::new().extend([
+            ("blah", vec![0]),
+            ("1a", vec![1]),
+            ("2", vec![2]),
+            ("0", vec![3]),
+        ]);
+
+        assert_eq!(vocabulary.len(), 4);
+        assert_eq!(vocabulary["blah"], &[0]);
+        assert_eq!(vocabulary["1a"], &[1]);
+        assert_eq!(vocabulary["2"], &[2]);
+        assert_eq!(vocabulary["0"], &[3]);
+    }
+}