Peptidylarginine deiminase (PAD, EC 3.5.3.15) enzymes catalyze the conversion of protein‐bound arginine to citrulline. This post‐translational modification may have a big impact on the structure and function of the target protein. In this review, we will discuss the effects of citrullination and its involvement in several human diseases, including rheumatoid arthritis and multiple sclerosis. So far, four isotypes of PAD have been described in mammals. We describe the existence of PAD in non‐mammalian vertebrates and the existence of a fifth mammalian PAD. In addition, tissue‐specific expression, genomic organization and evolutionary conservation of the different PAD isotypes will be discussed in detail. This article contains supplementary material which may be viewed at the BioEssays website at http://www.interscience.wiley.com/jpages/0265‐9247/suppmat/2003/25/v25.1106.html. BioEssays 25:1106–1118, 2003. © 2003 Wiley Periodicals, Inc.