This paper explores several theories of human development, with particular attention to the development of social interaction. Part 1 compares and contrasts major developmental theories, including those of Freud, Erikson, Piaget, Kohlberg, Kegan, Fowler, and Selman. From birth to 1 year, infants are laying the foundation that will guide their later social interactions. Between years 2 and 5 the beginnings of autonomy and the dominance of egocentrism can be observed. Children ages 6 to 12 show concrete thinking, the emergence of the self-concept, the freedom from impulses, and the need to be successful. Youth, age 13 and older, show many precursors of adult attitudes and behaviors, with identity formation as a major issue, the possibility of abstract thought, and the beliefs of the community as a source of strength. Part 2 applies those theories to the expanding understanding of friendship, emphasizing the friendships of school-age children. Self-acceptance plays a crucial role in social interaction development. In Stage 1 of friendships, birth to 2 years, children play in each other's presence rather than with each other. Stage 2, ages 2 to 5, involves quickly changing friendships, characterized by creativity, joint fantasy, and shared imagination. Stage 3, ages 6 to 12, entails the emergence of reciprocity, shared activities, increasing peer influence, and the separation of self-perception from social status. Stage 4, ages 13 and following, is characterized by the recognition that friends have rights and relationships that are independent of oneself, increasing stability of self-esteem, and true moral thinking. (Contains 14 references.) (KDFB)