This brief history of propositional functions shows that they are useful entities and that they have played a central role in logic as it is used in philosophy and linguistics. I have omitted the more mathematical uses of propositional functions, for example, in Russell's and Ramsey's constructions of classes, and in treatments of general models for higher-order logic. But the topic of propositional functions is a big one and we can't cover it all in a single encyclopedia article.