The first patellar dislocation is called acute or primary patellar dislocation, while a condition in which there is a tendency of patella escaping its anatomical position and in which repeated dislocations occur is referred to as chronic or recurrent patellofemoral instability. People aged between 10 and 17 are most commonly affected.. Patellofemoral joint is stabilized by static and dynamic stabilizers. Acute patellar dislocation is most commonly caused by an indirect knee trauma, while chronic patellofemoral instability is caused by an injury to the MPFL or by anatomical abnormalities like trochlear dysplasia, patella alta, increased TT-TG distance or rotation abnormalities of the lower extremities. Acute patellar dislocation is characterized by intense pain and hemarthrosis, while patellofemoral instability is characterized by the weakness in the knee, decrease in sporting activity, and osteoarthritis. Clinical assessment includes taking history and physical examination. In taking history, the most important things to find out are the mechanism of the injury, the symptoms that the patient experiences and if he had suffered from similar symptoms ever before. During the physical examination, it is important to test patellar stability, e.g. by looking for the J sign or by patellar apprehension test. Imaging includes conventional radiography, MRI and CT. Patellofemoral instability can be treated conservatively and surgically. Conservative treatment includes administration of the nonsteroidal anti-inflammatory drugs, RICE method, and physical therapy. Most commonly used surgical method is MPFL reconstruction, while trochleoplasty and distal realignment procedures are also commonly used.