The extensor mechanism of the knee is essential to ambulation and is subject to a number of traumatic, congenital, and inflammatory processes. In the pediatric population, the spectrum of pathologic conditions affecting the extensor mechanism is specific to skeletally immature patients. In addition, certain congenital and developmental disorders may further predispose the knee extensor mechanism to injury. The pathologic processes can be subdivided into categories: conditions of the attachments and insertions of the quadriceps and patellar tendons, conditions of the patella, conditions of the quadriceps muscle group, and avulsions of the superior attachments of the quadriceps. Cases of conditions affecting the extensor mechanism of the pediatric knee were collected at two large trauma centers, and the clinical and radiologic features were reviewed. Initial evaluation of these conditions is performed with radiography, but magnetic resonance imaging has evolved into a useful adjunct for assessing the soft tissues for a more precise evaluation of the true extent of an injury, thereby affecting decisions about surgical intervention and prognosis.