Hip and knee osteoarthritis (OA) are the most important because of the high prevalence of pain and disability that they cause in older adults, and the massive healthcare resource input that results from this, particularly in terms of the provision of joint replacements. This chapter describes joints and how diseases can affect them. It provides a discussion of 'common-or-garden' OA, and explains some of the atypical variants of OA. Lesser forms of neurological change, including weak muscles, and loss of proprioception, may be important in 'common-or-garden' OA. Some racial/ethnic differences exist in the prevalence and distribution of OA. Obesity is a strong risk factor, particularly for knee OA. The chapter illustrates the basic principles of the symptomatic management of OA. It also describes some of the conditions that can predispose to OA, as well as some interesting anatomical variants that differ in phenotype from 'common-or-garden' OA.