Pain on the front part of the leg that is felt deep in the center of the knee can often be from wear of this cartilage. This cartilage wear is called chondromalacia and the pain syndrome may be called patellofemoral syndrome (or PFS for short).
There are many causes for anterior knee pain. Overuse in work or exercise, sports injuries, arthritis, and even flat feet have been implicated in causing this type of pain. The symptoms are numerous and include pain with kneeling, "start-up" pain that occurs after sitting for too long, and the classic "theater" sign -- pain in the knee cap area while sitting in a cramped seat like in a movie theater. Sometimes the knee can "catch" or "grind" or even swell in certain situations. If there is swelling, the knee can feel warm, as well.
Treatment is aimed at quieting down the inflammation with ice/rest/elevation and sometimes using anti-inflammatory medications. These pills can have some side effects, so be sure to read the instructions and contact your PCP if you are unsure if they are right for you.
Exercises to strengthen the quadriceps muscle in the front of the thigh have also been shown to slowly decrease these symptoms. Sometimes a doctor will prescribe orthotics/inserts for your shoes if the alignment of the knee cap is out of balance. Formal physical therapy is reserved for long-term problems, and surgery is done rarely at this stage.
As always, if this pain persists or is associated with new symptoms or a new injury, be sure to talk to your PCP or orthopedic surgeon as further investigation may be necessary.