Among researchers who attempt to distinguish abusive sexual encounters between siblings from normal behavior, two variables are often mentioned: consent or, alternatively, coercion (intimidation, force, or violence) and the age difference between sex partners. De Jong (1989) has composed six questions that may assist the clinician in distinguishing sexual experimentation from exploitation; these questions concern age difference, developmental level, motivation, coercion, outside influence, and the individual's response to contact. The prevalence rates of sibling sexual abuse vary widely. Some of these differences can be explained by differences in the studies on which the rates are based. There are many estimates, and all are believed to be underestimates. Some of the "working underestimates" are that approximately 3 percent of the population has been sexually abused by a sibling, and approximately 12 percent of all incest is sibling incest. Although prevalence rates on sexual abuse between peers are minimal, evidence suggests that most childhood sexual encounters occur with a friend. Most of the sibling sexual abuse literature concerns the effects of such abuse. Characteristic of this literature is a schism between those who purport that the effects are negative and those who purport that they are positive or neutral. Some of the discrepancy in outcomes across studies can be explained by differences in the victims' ages, developmental stage, degree of coercion, and other variables. One relatively consistent finding is that generally negative effects stem from coercive sexual contact between siblings, cousins, and peers. Clinical implications of this literature review are discussed. 2 tables and 26 references