CD30L is a type II membrane glycoprotein belonging to the TNF Superfamily.In the case of CD30L, definition of its biological role remained frustratingly elusive. Early functional studies using CD30L⁺ cells or agonistic CD30^-specific antibodies logically focused attention on cell types that had been shown to express CD30, namely certain lymphoid malignancies and subsets of activated T cells. However, it was not immediately clear how the reported activities from these in vitro studies relate  to the biological activity of CD30L in the more complex whole animal setting. Recently, results from in vivo models involving CD30 or CD30L gene disruption, CD30L overexpression, or pharmacological blockade of CD30/CD30L interactions have begun to provide clues about the role played by CD30L in immunological processes.