Instability of the patella (kneecap) is one of the most common reasons patients seek medical treatment at orthopaedic and sports medicine clinics. Normally, the patella glides smoothly and stays within what is termed the trochlear groove as the knee flexes (bends) and extends (straightens). An unstable kneecap comes either completely or partially out of the groove as the knee bends. The term “patellar instability” may indicate either a dislocation, where the kneecap comes completely out of its normal position, or a subluxation, where the kneecap only partially moves out and then goes back into its normal position. There are many potential causes of patellar instability, ranging from a traumatic dislocation injury to inherent problems with the patient’s anatomy that predisposes them to this problem. Acute patellar dislocation is a common traumatic knee injury in children and teenagers. At least one-half of patellar dislocations occur during sports activities such as basketball, football, and soccer. Once this injury occurs, the chance of sustaining future dislocation injuries ranges from 14-57% in adult patients and 36-71% in children and teenagers. Fortunately, most patients who suffer first-time dislocations do not require surgery. However, there are some instances where an operation is necessary soon after the injury and this topic is discussed in detail in this eBook. Chronic, or recurrent, patellar instability may occur for many reasons and may or may not happen as a result of a previous dislocation injury. There are many factors which affect patellar stability such as the angle of knee flexion, the shape (geometry) of the bones in the knee joint (trochlear groove and lateral femoral condyle), tissues referred to as static patella stabilizers, and tissues called dynamic patella stabilizers. An abnormality, weakness, or imbalance in these factors may cause recurrent patellar dislocation or subluxation episodes. These persistent problems may result in kneecap pain, damage to the joint lining underneath the kneecap, loss of the ability to participate in sports activities, and even difficulty with activities such as squatting, kneeling, and going up and down stairs. If not treated appropriately, these repeat injuries may result in eventual arthritic damage to the undersurface of the kneecap and the trochlear groove. While conservative treatment (physical therapy, support brace, weight control) frequently helps alleviate symptoms, surgery may become necessary in some patients to correct all of the anatomic problems that are causing the instability problems. Dr. Frank Noyes, an internationally renowned orthopaedic surgeon, and Sue Barber-Westin, Director of Clinical Research at the Cincinnati SportsMedicine Research Foundation, team up to provide information that is easy to read and understand regarding patellar instability. This eBook provides information on basic knee anatomy, how the knee and lower limb should work to keep the patella stable, the potential effects of patellar instability, treatment options for acute dislocations and recurrent subluxations, when surgery is necessary, different types of operations that are commonly done, and exercises to help improve muscle strength and flexibility.