What commonly causes knee pain?

Knee pain: Knee pain can be caused by injury, overuse, arthritis, cancer, hip or spine issues and other things. To help your physician understand your specific pain, you want to write down your medical problems and medicines. Spend some time thinking about what causes your pain, what makes it better and worse. Record any other findings associated with your knee pain. (ie swelling, popping, giving way...)

Is it front of knee?: Movement at the knee joint is facilitated by the knee cap and powerful thigh muscles (patello-femoral complex). In the absence of major injury (which can cause ligament sprain / cartilage tears) a common cause of knee pain in younger adults is anterior, or patello-femoral pain. See http://patient.info/health/knee-pain-patellofemoral-pain for a sensible summary on this.