Just as bodily actions are things you do with your body, mental actions are things you do with your mind. Both are different from things that merely happen to you. Where does the idea of mental action come from? What are mental actions? And why do they matter in philosophy? These are the three main questions answered in this paper. Section 1 introduces mental action through a brief history of the topic in philosophy. Section 2 explains what it is to (...) be a mental action in terms of intentional action. Section 3 argues that the fact that we can perform mental actions has some striking consequences concerning mental content. Section 4 shows how mental action helps to explain some important facts about self-knowledge and the normativity of thought. The conclusion mentions two further fruitful avenues for philosophical research on mental action. (shrink)