How young children come to seek out other human beings and how they communicate effectively with them have been topics of considerable import for philosophers, scientists, and parents from at least the mid-19th century. Dramatic changes take place in these domains during the first 3 years of life, changes that provide both clear opportunities and challenges for our understanding of developmental processes. Sociability is seldom attributed to human newborns, who although they are selectively responsive to forms of stimulation that other humans provide par excellence, appear not to understand that this stimulation comes from another, separate being. Yet, as early as 1 year of age, infants clearly recognize the many familiar human beings of their world, seek out interactions with one or more of these familiar humans, and actively protest and seek to redress forced separations from them. Further, even though newborns are engaged in communication and cooperative interactions with other human beings from birth, the forms of cooperative action, infants' roles in generating them, and the forms of communication employed change markedly over the first 3 years. In the Western industrialized societies most intensively studied, the forms of cooperative action change from the suckling bouts and en face exchanges of affective signals characteristic of the first months after birth to the 3-year-old's newly generated games, collaborative pretend play, and verbal conversations and arguments.