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The conceptualization of factors responsible for the etiology of patellofemoral pain are changing. In the recent past, chondromalacia and malalignment were believed to be most important factors relative to the genesis of anterior knee pain symptoms. However, new evidence from Europe and North America indicates other factors may be more important, such as overuse of anatomically normal patellofemoral structures, leading to the possible loss of both osseous and soft tissue homeostasis. The range of painless loading compatible with tissue homeostasis of a joint without causing structural or physiologic injury is termed the “envelope of function.” Restoration of the envelope of function as safely and predictably as possible is viewed as the primary underlying principle of treatment for patients with patellofemoral pain.