Castleman disease (CD) describes a group of at least four disorders that share a spectrum of characteristic histopathological features but have a wide range of etiologies, presentations, treatments, and outcomes. CD includes unicentric CD (UCD) and multicentric CD (MCD), the latter of which is divided into idiopathic MCD (iMCD), HHV8-associated MCD (HHV8-MCD), and POEMS-associated MCD. iMCD can be further sub-classified into iMCD-TAFRO (thrombocytopenia, ascites, reticulin fibrosis, renal dysfunction, organomegaly) or iMCD-NOS (not otherwise specified), who often have thrombocytosis and hypergammaglobulinemia. Advances in diagnosis, classification, pathogenesis, and therapy are substantial since the original description of UCD by Benjamin Castleman in 1954. The advent of effective retroviral therapy and use of rituximab in HHV8-MCD have improved outcomes in HHV8-MCD. Anti-interleukin-6 directed therapies are highly effective in many iMCD patients, but additional therapies are required for refractory cases. Much of the recent progress has been coordinated by the Castleman Disease Collaborative Network (CDCN), and further progress will be made by continued engagement of physicians, scientists, and patients, the last of which can be facilitated by encouraging patients to self-enroll in the CDCN's ACCELERATE natural history registry (#NCT02817997, www.CDCN.org/ACCELERATE).