Most common in under 18s- Patellofemoral pain (usually adolescents), Osgood-Schlatter's disease, referred pain from hip (e.g.. slipped capital femoral epiphysis), joint hypermobility syndrome, growing pains, patellar subluxation/instability, osteochondritis dissecans, septic arthritis, tumours, patellar tendonitis, bipartite patella, Henoch-Schonlein purpura