Although sex play is part of normal childhood development, some sexual activity may be coerced or forced and may have negative long-term consequences. Studies show prevalence rates of sibling sexual abuse vary widely, very few victims disclose their abuse, and sibling sexual abuse is underestimated. Prevalence estimates indicate 3 percent of the population has been sexually abused by a sibling, 67 percent of sibling abuse involves sexual abuse, and 12 percent of all incest involves a sibling. Available literature on sibling child sexual abuse is critically reviewed, and particular attention is paid to defining sibling sexual abuse and determining its scope and effects. Clinical implications are derived as well. Even though the article focuses on sibling sexual abuse, concepts considered in the literature review also apply to other forms of same-age child sexual relationships. The author concludes practitioners should consider the possibility that their patients have been sexually abused as children by siblings, cousins, or peers. 26 references and 2 tables