The present study set out to determine the influences that family members, other children, and other adults have on theory of mind development by examining their impact on false belief understanding and on engagement in pretend play, as well as to examine the relationship between pretend play and false belief understanding. Both standard and family-situated false belief tasks were given to 71 children between the ages of 3;4 and 5;0 (M = 4;0). There were no differences in performance between these types of tasks; children were very consistent in either failing or passing all six false belief tasks in this study. The impact of having siblings was examined, and older children were found to gain an advantage by having older siblings, but not younger siblings, while the younger children did not gain an advantage by either older or younger siblings. Both increased age and language ability were related to false belief success. When these were controlled for, children's pretend play, and specifically their pretend play with other children, was positively related to their false belief performance, while non-pretend play and solo pretend play were not. Overall interactions with others, including both pretend and non-pretend activities were not predictive of false belief success, once age and language were controlled for, but children's non-pretend play with parents, including reading, watching television, and playing games, was positively associated with false belief performance. A regression equation was performed to account for the variance in false belief success, and the following variables each contributed significantly in reducing variance: age, language ability, non-pretend play engaged in with parents, pretend play engaged in with other children, and play involving imaginary companions and impersonation of people, characters, or animals. This study provides evidence that individual differences are indeed related to development of false belief understanding.