The sacroiliac (SI) joint is a common source of low back pain in the general population. Because it is the link between the lower extremities and the spine, it sustains even higher loads during athletic activity, predisposing athletes to a greater probability of joint dysfunction and pain. The diagnosis and treatment of SI joint dysfunction remains controversial, due to complex anatomy and biomechanics, and a lack of universally accepted nomenclature and terminology, consistently reliable clinical tests and imaging studies, and consistently effective treatments. This article clarifies these issues by presenting a model of SI joint anatomy and function, a systematic approach to the diagnosis of dysfunction, and a comprehensive treatment plan.