Instability of the patella (kneecap) is a common reason patients seek medical treatment. "Patellar instability" indicates 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. This problem may occur from a traumatic dislocation injury or due to inherent problems with a patient's anatomy. Dr. Frank Noyes, an internationally renowned orthopaedic surgeon, and Sue Barber-Westin, Director of Clinical Research at the Cincinnati SportsMedicine Research Foundation, provide information that is easy to read and understand regarding patellar instability. This eBook describes basic knee anatomy, how the knee and lower limb should work to keep the patella stable, the potential effects of patellar instability, treatment options, when surgery is necessary, different types of operations that are commonly done, and exercises to help improve muscle strength and flexibility.