Exercise is extremely important for overall musculoskeletal health and this is especially true for the knee area. The knee joint benefits from both general overall body exercise and fitness programs, as well as specific exercises for the knee itself (strength, flexibility, ROM) and its surrounding musculature.
General exercise and fitness programs help control weight and prevent obesity, and also strengthen important muscle groups that protect the knee. This includes some exercises that are more obvious in terms of knee function like the quadriceps or thigh muscle, but also other muscle groups that help protect against knee injuries that are not as obvious such as the core and abdominal area as well as certain pelvic related muscles.
Individuals with certain knee problems often need very specific customization of their exercise program, something I deal with in great detail in my book FrameWork for the Knee. This is especially true for those with patella (kneecap) as well as those with arthritis. There are also some tremendous exercise routines that can help prevent anterior cruciate ligament tears, a devastating injury for athletes young and old. I would encourage anyone with knee problems (or those that want to prevent them) to check out FrameWork for the Knee.