Theory of mind is said to be uniquely human. Is this statement justified? Thirty years of research on a variety of species has produced differences in opinion, from unequivocal positive evidence to no evidence at all for mental attribution in animals. Our review concludes that animals are excellent ethologists, but on the whole, poor psychologists. Those studies that we believe present a good case for mental attribution all possess high ecological validity, including studies on food competition by chimpanzees and cache-protection strategies by corvids. Even though the current focus of research on prediction rather than explanation may be misplaced, we believe the field is now in a strong position to discover what animals really know about their fellow beings, be it based on simple associations, behavior reading, mind reading, or something else.