The chapter focuses on the behavioral line of children’s sexual development. (For purposes of this chapter, the term "children" refers to prepubertal children.) It outlines seven lines of sexual development in children: the biological; the change from sensual to erotic; the behavioral; gender role and gender object-choice; cognitive understanding; sexual relationships; and sexual socialization. It provides a definition of natural and healthy sexuality in childhood that functions as a baseline for understanding children’s sexual behaviors. It presents 21 characteristics of problematic sexual behavior and delineates three groups or patterns of problematic behaviors. It provides case examples involving children in each of those groups. Children’s sexual development begins in utero and continues through puberty and into adulthood. Children’s sexual behavior is affected both positively and negatively by their environment and by specific experiences and the context of those experiences. Children’s sexual behaviors can be categorized as sexual play, sexually reactive behavior, extensive mutual sexual behaviors, and sexually abusive behavior (molestation), with a range of behaviors within each group. Table, references