Since its initial description, researchers have expanded the spectrum of Castleman disease to include not only the classic and well-recognized hyaline-vascular type, but also the plasma cell type and multicentric types of broader histologic range, including human herpes virus-8-associated Castleman disease. These less common subtypes of Castleman disease are less familiar, and may be under-recognized. Also of practical importance, current authors are restructuring the classification of multicentric Castleman disease to accommodate the emerging pathogenic role of human herpes virus-8 and its association with the recently described plasmablastic variant. In addition to an increased risk of lymphoma, patients with Castleman disease also are at increased risk for other related neoplasms, including Kaposi sarcoma and follicular dendritic cell tumors, which are of prognostic and therapeutic relevance. This review focuses on the histologic diagnosis of Castleman disease, current and emerging concepts in its pathogenesis and classification, and associated histopathologic entities.