The purpose of this cross-national descriptive study is to measure the extent of children's sexual knowledge and sexual understanding at various ages and to identify what processes of thought children use in trying to explain biological functions and the phenomena of their own bodies as they grow and change. Sexual thinking is defined as thinking about that broad area of sex and sexuality which impinges upon the child's world from birth. Face-to-face individual clinical interviews were devised to apply to children 5, 7, 9, 11, 13, and 15 years of age. Administered in Australia, England, North America, and Sweden, the interviews covered six areas: (1) aging and the best time to be alive in the human life sequence; (2) parents' identities and roles as mothers and fathers and as men and women; (3) children's sex preferences and the way in which children perceive sex differences in the newborn and during puberty; (4) the origin of babies and the role of mothers and fathers in procreation, gestation, birth, and related processes, including the area of "not having babies"; (5) children's ideas about sex education at home and at school; and (6) clothing and nakedness. Children were also asked to define words on a sexual vocabulary list. Results are discussed in reference to developmental theories of childhood and implications for sex education. (RH)