With mutable state, your function mutates state somewhere in the environment (typically a global variable or the class in which the function resides). With immutable state, your function returns an immutable value, never modifying anything. I program Java for the most part, and this is how I do most things. Mutable state is the enemy, not OOP. Some people think mutable state and OOP are inseparable but they are just looking at a definition of it that is easy to criticize. If you make every object immutable , OOP actually becomes a much nicer model to work with.