The protein arginine deiminases (PADs), and in particular PAD4, have emerged as potential therapeutic targets for the treatment of rheumatoid arthritis (RA). In this review, evidence linking dysregulated PAD activity to the onset and progression of RA is presented, and the potential role of such aberrant activity in other human diseases, such as multiple sclerosis and cancer, is discussed. The known physiological roles of the PADs, particularly PAD4, and current knowledge regarding PAD structure, catalysis and inhibition are also described.