New knowledge about the pathophysiology and biomechanics of the patellofemoral joint (PFJ) has led to a better understanding and new treatment options. To recognise the main pathology is the most important and difficult part. This article describes the basic principles of biomechanics (static, passive and active factors) and their influence on the PFJ. Patients are classified into three groups (pain, instability and arthritis). This is achieved by a clinical evaluation and standard X-rays. The instability group is analysed additionally with MRI. The aim of this evaluation is to analyse the biomechanical cause of the instability. Based on a pathophysiological concept a new therapeutic algorithm is introduced.