This article is an introduction to the Special Issue. Reactive Attachment Disorder (RAD) has been in the psychiatric nosology since 1980, when DSM-III was published. Since 1980, however, there has been little research on RAD as a diagnostic category. On the other hand, research on the development of patterns of attachment has informed clinical practice. In this article, selected diagnostic issues are reviewed and a model for conceptualizing the continuum of levels of attachment adaptation in infants and young children is presented. The articles in this Special Issue are introduced and the issues they address are highlighted.