Arthritis mutilans is described as the most severe form of psoriatic arthritis. It is characterized by digital shortening associated with severe osteolysis of peripheral joints. Research on arthritis mutilans has been impeded by lack of an accepted case definition. Here we describe the definitions used to date, epidemiology, clinical and radiological features, and clues to pathogenesis of arthritis mutilans. An initiative by the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) to develop a consensus definition is described. The purpose of the GRAPPA initiative is to assist studies identifying clinical predictors and biomarkers for arthritis mutilans, so that patients at risk are identified early and appropriate therapeutic intervention is instituted to prevent joint destruction and preserve quality of life and function.