CD30, also known as TNFRSF8, is a cell membrane protein of the tumor necrosis factor receptor family, which regulates proliferation/apoptosis and antibody responses. CD30 is expressed by activated, but not by resting, T and B cells. Aberrant expression of CD30 by mastocytosis mast cells and interaction with its ligand CD30L (CD153) appears to play an important role in the pathogenesis and clinical presentation of systemic mastocytosis. CD30 has been considered as a specific diagnostic biomarker of anaplastic large cell lymphoma (ALCL) and classical Hodgkin lymphoma (cHL). CD30 is also a biomarker used for targeted therapy by an antibody–drug conjugate. Soluble CD30 retains the ability to bind CD30 Ligand and functions as an inhibitor of normal CD30 signaling.