Age is key to the differential diagnosis – the very young (developmental dysplasia), school-age child (Perthes disease or irritable hip / transient synovitis) and in the older child (Slipped Upper [Capital] Femoral Epiphysis). At all ages, consider red flag conditions (sepsis, malignancy, non-accidental injury). Rheumatic fever and chronic low grade infection such as TB will also need to be considered in at risk populations.