Canine immune-mediated polyarthritis (IMPA) is a diagnosis of exclusion based predominantly on clinical signs, characteristic joint fluid analysis, and elimination of potential joint infection. Ultimately, an appropriate and sustained response to immunosuppressive therapy may become the final diagnostic criterion used. Identifying associated disease processes, including breed-specific syndromes, remote infection, inflammation, drug exposure, vaccine exposure, or neoplasia, as well as initial response to therapy, is often an important contributor to prognosis. This review article is the second of a two part series and focuses on the diagnosis and treatment of immune-mediated polyarthritis. The first article in this series, published in the January/February 2012 issue, concentrated on the pathophysiology of IMPA.