This paper is about a puzzle which lies at the heart of contemporary physicalist theories of mind. On the one hand, the original motivation for physicalism was the need to explain the place of mental causation in the physical world. On the other hand, physicalists have recently come to see the explanation of mental causation as one of their major problems. But how can this be? How can it be that physicalist theories still have a problem explaining something which their physicalism was intended to explain in the first place? If physicalism is meant to be an explanation of mental causation, then why should it still face the problem of mental causation?