This introductory article provides an overview of the significant issues involved when dealing with youth who sexually offend, sometimes referred to as juvenile sex offenders or sexually reactive children or adolescents. There is not an accepted term or definition that is widely used to describe or refer to this population, and the precise prevalence or incidence rates are not known. Statistics are presented from various national studies, but methodological problems in the research are also noted. The authors briefly discuss the current research concerning youth who sexually offend, present some of the important issues in this area of research and practice, and list various types of sexual victimization that have been included when dealing with youth who sexually offend. The article then introduces the current volume, describing the articles and content. Identifying and treating youth who sexually offend is in its infancy in many ways. It is hoped that this volume will provide important information to help those in research and practice better understand the issues and dynamics of this population.