Children use language to achieve social goals, particularly to express reasons for their beliefs and to understand others’ reasons for their beliefs. Although reasoning is traditionally viewed as an individual skill, recent accounts have focused on the social dimension of the process and the exchange of reasons/justifications in communication to reach joint decisions and solve problems. The literature on argumentation often focuses on school-aged children’s reasoning abilities. Recent research, however, shows that much younger children (e.g., children around age 3), begin to communicate reasons effectively to achieve their social goals in interaction. The entry highlights that the context of cooperative problem solving with same-age peers as a unique context that facilitates young children’s reasoning.