The knee is the most affected joint in children with cerebral palsy. Flexion contracture of the knee is the cause of crouch gait pattern, instability in stance phase of gait, and difficulties during standing and sitting, and for daily living activities. It may also cause patella alta, degeneration of the patellofemoral joint, and stress fractures of the patella and tibial tubercle in young adults. Children with cerebral palsy may even give up walking due to its high energy demand in the adult period. The purpose of this article is to review the causes of the knee flexion contractures, clinical and radiological evaluations, and treatment principles in children with cerebral palsy. The biomechanical reasons of knee flexion deformity are discussed in detail in the light of previous Studies and gait analysis data.