Children use language to achieve social goals, particularly to express reasons for their beliefs and 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 (Mercier & Sperber, 2017; Tomasello, 2018). The literature on argumentation, however, often focuses on school-aged children’s reasoning abilities. This chapter lays out how much younger children, children around age 3, begin to communicate reasons effectively to achieve their social goals in interaction. We will highlight that the context of cooperative problem solving with same-age peers is unique in facilitating young children’s reasoning.