Humans have the capacity for awareness of many aspects of their own mental lives—their own experiences, feelings, judgments, desires, and decisions. We can often know what it is that we see, hear, feel, judge, want, or decide. This article examines the evolutionary origins of this form of self-knowledge. Two alternatives are contrasted and compared with the available evidence. One is first-person based: self-knowledge is an adaptation designed initially for metacognitive monitoring and control. The other is third-person based: self-knowledge depends on the prior evolution of a mindreading system which can then be directed toward the self. It is shown that the latter account is currently the best supported of the two.