diff --git a/package.yaml b/package.yaml
index 753a614..923dbe3 100644
--- a/package.yaml
+++ b/package.yaml
@@ -21,6 +21,9 @@ description:         Please see the README on GitHub at <https://github.com/gith
 
 dependencies:
 - base >= 4.7 && < 5
+- transformers >= 0.5.6 && < 0.6
+- containers >= 0.6 && < 0.7
+# algebraic-graphs >= 0.4 && < 0.5
 
 library:
   source-dirs: src
diff --git a/src/Rextra/Dfa.hs b/src/Rextra/Dfa.hs
new file mode 100644
index 0000000..e1312ee
--- /dev/null
+++ b/src/Rextra/Dfa.hs
@@ -0,0 +1,36 @@
+module Rextra.Dfa where
+-- TODO don't export internals
+
+import qualified Data.Map.Strict as Map
+import qualified Data.Set as Set
+
+data State s t = State
+  { transitions       :: Map.Map t s
+  , defaultTransition :: s
+  , accepting         :: Bool
+  }
+
+data Dfa s t = Dfa
+  { dfaStates     :: Map.Map s (State s t)
+  , dfaEntryState :: s
+  }
+
+{-
+ - Constructing and modifying a DFA
+ -}
+
+-- TODO
+
+{-
+ - "Executing" a DFA
+ -}
+
+getState :: (Ord s) => Dfa s t -> s -> State s t
+getState dfa s = dfaStates dfa Map.! s
+
+transition :: (Ord s, Ord t) => Dfa s t -> s -> t -> s
+transition dfa s t =
+  let state = getState dfa s
+  in  case transitions state Map.!? t of
+        (Just nextState) -> nextState
+        Nothing          -> defaultTransition state
diff --git a/src/Rextra/Nfa.hs b/src/Rextra/Nfa.hs
new file mode 100644
index 0000000..f26da9f
--- /dev/null
+++ b/src/Rextra/Nfa.hs
@@ -0,0 +1,45 @@
+module Rextra.Nfa where
+-- TODO don't export internals
+
+import qualified Data.Map.Strict as Map
+import qualified Data.Set as Set
+
+data TransitionCondition t
+  = Only (Set.Set t)
+  | AllExcept (Set.Set t)
+
+specialStates :: TransitionCondition t -> Set.Set t
+specialStates (Only s)      = s
+specialStates (AllExcept s) = s
+
+accepts :: (Ord t) => TransitionCondition t -> t -> Bool
+accepts (Only s)      t = Set.member t s
+accepts (AllExcept s) t = Set.notMember t s
+
+type State s t = [(TransitionCondition t, s)]
+
+data Nfa s t = Nfa
+  { nfaStates     :: Map.Map s (State s t)
+  , nfaEntryState :: s
+  , nfaExitStates :: Set.Set s
+  }
+
+{-
+ - Constructing and modifying a NFA
+ -}
+
+-- TODO
+
+{-
+ - "Executing" a NFA
+ -}
+
+getState :: (Ord s) => Nfa s t -> s -> State s t
+getState nfa s = nfaStates nfa Map.! s
+
+-- * Starting from a state, find all the states that it can transition to with token 't'.
+nextStates :: (Ord s, Ord t) => State s t -> t -> Set.Set s
+nextStates state t = Set.fromList . map snd . filter (\(cond, _) -> cond `accepts` t) $ state
+
+nfaTransition :: (Ord s, Ord t) => Nfa s t -> Set.Set s -> t -> Set.Set s
+nfaTransition nfa ss t = foldMap (\s -> nextStates (getState nfa s) t) ss