The presence of an intra-articular knee effusion requires an extensive differential diagnosis and a systematic diagnostic approach. Pediatric knee effusions occur most commonly as acute hemarthroses after traumatic injury. However, the knee joint is susceptible to effusions secondary to a wide variety of atraumatic causes. Special attention is required in the atraumatic effusion to distinguish features of infectious, postinfectious, rheumatologic, hematologic, vasculitic, and malignant disease. This review discusses the various etiologies of both traumatic and atraumatic pediatric knee effusions highlighting the historical, physical examination, and laboratory characteristics to aid the emergency provider in diagnosis and initial management.