Child sexual abuse is any sexual act imposed on a child, and incest is such an act by any immediate or extended family member. Sexually abusive behaviors include disrobing, genital exposure, observing the child's nudity, kissing, fondling, masturbation, fellatio, cunhilungus, penetration of the rectal opening with the finger, and vaginal penetration. The sexual activity usually occurs in five separate phases. During the engagement phase, the perpetrator uses persuasion to involve the child in some type of sexual behavior. The sexual interaction phase generally involves more extensive sexual contact. After the sexual contact has occurred, the perpetrator's main task is to commit the child to secrecy. The fourth phase is the disclosure phase, which may entail accidental disclosure or purposeful disclosure. Following disclosure, most cases enter a suppression phase, in which the family attempts to suppress publicity, information, and intervention. Perpetrators are usually satisfying many nonsexual needs. The incest victim's mother is usually in a subordinate position to the perpetrator. Child victims should never be held responsible for the sexual activity. The incestuous family regards outsiders as threatening and fosters a pathological dependence among family members. Sexual abuse is nearly always a profoundly disruptive, disorienting, and destructive experience for the child. Clinicians working with victims need a basic knowledge of the mechanics and dynamics of child sexual abuse in order to have a basis for effective intervention. Case histories are provided, and five references are listed. For the book of which this paper is a chapter, see NCJ 97363.