Theory of mind, the ability to attribute mental states to oneself and others, has traditionally been investigated in humans and nonhuman primates. However, non-primate species, such as domestic dogs, may also be potential candidates for such a faculty. Domestic dogs (Canis familiaris) evolved from a social-living, wolf-like ancestor, and were the first species to be domesticated, with likely selection for sensitivity to human cues and human-like cognitive abilities. Dogs typically spend their lives in the rich social environment of human families, and thus dogs are naturally enculturated. The combination of these factors make dogs an excellent candidate for having a functional theory of mind. Yet perhaps surprisingly, prior research on theory of mind in dogs is limited, with inconclusive and contradictory results. The research described in this thesis is a systematic investigation of dogs' potential to demonstrate a functional theory of mind in their interactions with humans. Four experiments are presented, based on the Knower-Guesser paradigm (Povinelli et al., 1990), in which a knowledgeable and an ignorant human informant indicated the location of hidden food to the dog. In Experiment 1, one informant was absent (Guesser) and one present (Knower) during the food-hiding, and the dogs chose the Knower. However, when both informants were present, the dogs chose the informant that did the baiting, but this preference was less than when the Guesser was absent. In Experiments 2 and 3, a third experimenter hid the food while the informants covered their cheeks (Knower) or eyes (Guesser) with their hands, or were attentive (Knower) or inattentive (Guesser) to the food-hiding. In both cases, the dogs showed a significant preference for the Knower. In Experiment 4, the dogs showed no preference between the informants when they had equal perceptual access to the baiting, and were unsuccessful at selecting any container when the informants did not provide communicative cues. Overall, the present research provides the most definitive evidence yet that domestic dogs may be able to attribute differential states of knowledge to human observers, and thus may possess a functional theory of mind.