Calcium hydroxyapatite crystal deposition disease is characterized by the presence of basic calcium phosphate crystals--predominantly hydroxyapatite--in the periarticular soft tissues, especially the tendons. The entity is best recognized as "calcific tendinitis" at its most frequent site about the shoulder, but the disease involves numerous other sites and may be more appropriately termed calcific periarthritis. This article illustrates typical and atypical radiographic features of hydroxyapatite deposition, as well as some unusual manifestations of the disease. A brief review of various theories on the pathogenesis of the disease and a section on differential diagnosis is included.