Epidemiological data indicate a link between rheumatoid arthritis (RA) and periodontal disease (PD). In vitro and in vivo studies have sought to dissect potential mechanisms by which PD may contribute to initiation and progression of RA. However, these are both multifactorial, chronic diseases, and their complex etiologies and pathogenesis themselves remain incompletely understood. Could there really be an etiological link or does this simply represent a statistical coincidence muddied by common risk factors? This review seeks to provide background on these two diseases in the context of recent discoveries suggesting that their pathogenesis may be related. In particular, the process of citrullination, a post-translational protein modification, has been highlighted as a process common to both diseases. The evidence for a relationship between the diseases is explored and its potential mechanisms discussed.