Idiopathic thrombocytopenic purpura (ITP, also known as immune thrombocytopenic purpura) affects both children and adults. In childhood, the peak age is 2-4 years, girls and boys are equally affected, and in most children, the disease is self-limited with spontaneous recovery occurring in several weeks to several months. In adults, ITP is most common among young women, and the disease is more insidious in its onset and chronic in its course. In both children and adults, there are important unresolved issues in diagnosis and management that have major cost implications. For this reason, the American Society of Hematology chose ITP as the disease topic for its initial practice guideline in 1993. In 1996, the practice guideline was published in Blood, the Journal of the American Society of Hematology. This review describes the principle features of ITP in children and adults, the process of development of the practice guideline for ITP, and the implications of the guideline for the diagnosis and management of patients with ITP.