A limp is a deviation from normal gait pattern, with pain as the presenting feature in about 80% of cases. The differential diagnosis is broad and includes congenital/developmental, infectious, inflammatory, traumatic (including nonaccidental), and, less commonly, neoplastic etiologies. Transient synovitis of the hip is the cause of a limp in the absence of trauma in 80% to 85% of children. It can be differentiated from septic arthritis of the hip by the absence of fever or ill-appearance and with laboratory testing that shows normal or only mildly elevated inflammatory markers and white blood cell count. If septic arthritis is suspected, joint aspiration should be performed urgently with ultrasound guidance and the aspirated fluid sent for Gram staining, culture, and cell count. Patient history, such as breech presentation at birth, and a leg-length discrepancy on physical examination may suggest developmental dysplasia of the hip. Pain reported primarily at night can occur with neoplasms. Hip pain in an adolescent who is overweight or has obesity may suggest slipped capital femoral epiphysis. Knee pain in an active adolescent may suggest Osgood-Schlatter disease. Radiography shows the degenerative femoral head changes in Legg-Calvé-Perthes disease. Abnormalities in bone marrow shown on magnetic resonance imaging indicate septic arthritis. A complete blood count with differential, erythrocyte sedimentation rate, and C-reactive protein should be obtained if infection or malignancy is suspected.