Single café-au-lait macules (CALMs) are common in the pediatric population and in most children represent a normal finding. It is important to recognize whether the presence of multiple CALMs in a particular patient is normal or indicates an association with a multisystem disorder. This article addresses issues concerning the prevalence, genetics, and natural history of CALMs in the general population and reviews disorders in which CALMs are present as a characteristic trait. (J Am Acad Dermatol 1999;40:877-90.) Learning Objective: At the completion of this learning activity, participants should be familiar with the clinical and histologic aspects of café-au-lait macules and the prevalence, genetics, and natural history of them, particularly in children.