Over the past several decades, researchers have become increasingly interested in understanding how primates understand the behavior of others. One open question concerns whether nonhuman primates think about others? behavior in psychological terms, that is, whether they have a theory of mind. Over the last ten years, experiments conducted on the free-ranging rhesus monkeys (Macaca mulatta) living on Cayo Santiago have provided important insights into this question. In this review, we highlight what we think are some of the most exciting results of this body of work. Specifically we describe experiments suggesting that rhesus monkeys may understand some psychological states, such as what others see, hear, and know, but that they fail to demonstrate an understanding of others? beliefs. Thus, while some aspects of theory of mind may be shared between humans and other primates, others capacities are likely to be uniquely human. We also discuss some of the broader debates surrounding comparative theory of mind research, as well as what we think may be productive lines for future research with the rhesus macaques of Cayo Santiago. Am. J. Primatol. 78:106?116, 2016. ? 2016 Wiley Periodicals, Inc.