Oh, and almost forgot, you can check out lots of examples of this not only in the mtl, but also on the (old) Haskell Wiki. I've written a lot of simple (sometimes trivial) examples for people to look at Unique values -- very simple http://www.haskell.org/hawiki/MonadUnique A supply of values, a slight generalisation of the above http://www.haskell.org/hawiki/MonadSupply Random number generation http://www.haskell.org/hawiki/MonadRandom A state monad transformer with undo/redo http://www.haskell.org/hawiki/MonadUndo Last but not least, check out my Sudoku solver (everyone has one of these nowadays). I wrote it by constructing a special monad, in which the problem becomes really easy to solve. (Basically, a state monad which enforces the sudoku rules, and handles nondeterminism automatically). http://www.haskell.org/hawiki/SudokuSolver hope this all helps :) - Cale