A central theme of Sergio Pellis’ research program has been on the role of rough-and-tumble play in the development of social skills. Recently, Sergio and his colleagues have shown in rats that the experience of play in the juvenile period alters the anatomy of the brain cells of the prefrontal cortex, a part of the brain involved in mediating social skills. The next big question on his research horizon is to determine whether social systems that foster interactional social ambiguity have coopted rough-and-tumble play as a vehicle to foster and refine the development of nuanced social skills. This endeavor involves a comparative analysis of play across the order Primates, with the ultimate goal being to identify how humans evolved the peculiar patterns of social play that they have (e.g., sociodramatic, fantasy). Sergio has authored 195 journal articles and book chapters, and two books, one of which, The Playful Brain (2009), brings together over 30 years of study on play behaviour.