 Although one might find precursors to causal theories of mental content scattered throughout the history of philosophy, the current interest in the topic was spurred, in part, by perceived inadequacies in “similarity” or “picture” theories of mental representation. Where meaning and representation are asymmetric relations—that is, a syntactic item “X” might mean or represent X, but X does not (typically) mean or represent “X”—similarity and resemblance are symmetric relations. Dennis Stampe (1977), who played an important role in initiating contemporary interest in causal theories, drew attention to related problems. Consider a photograph of one of two identical twins. What makes it a photo of Judy, rather than her identical twin Trudy? By assumption, it cannot be the similarity of the photo to one twin rather than the other, since the twins are identical. Moreover, one can have a photo of Judy even though the photo happens not to look very much like her at all. What apparently makes a photo of Judy a photo of Judy is that she was causally implicated, in the right way, in the production of the photo. Reinforcing the hunch that causation could be relevant to meaning and representation is the observation that there is a sense in which the number of rings in a tree stump represents the age of the tree when it died and that the presence of smoke means fire. The history of contemporary developments of causal theories of mental content consists largely of specifying what it is for something to be causally implicated in the right way in the production of meaning and refining the sense in which smoke represents fire to the sense in which a person’s thoughts, sometimes at least, represent the world.