Your knees have lots of moving parts, and you use them a lot, so lots of things can go wrong. Too much of one kind of motion, especially if you don’t work up to it, can lead to “overuse” injuries. Simple wear and tear is a problem, especially as you age. Accidents can crack bones and tear tissue. With some conditions, your body attacks its own joints. Your doctor can help you sort out what’s going on with your knee when it doesn’t feel right.

If your knee hurts intensely after a bump, bang, or fall, you may have broken one of the bones that meet up there — the thigh, shin, and kneecap — or shifted one out of place. Go to the emergency room or see a doctor as soon as possible. Sometimes fractures happen more slowly, causing tiny cracks at the ends of the leg bones. This can happen when you’ve started using your knee more.

You hear a pop and can’t move after you suddenly change direction — often while playing soccer, football, or basketball. You may have torn your ACL, which connects the femur and the tibia and prevents the tibia from moving too far forward. Your knee will hurt and swell and feel unstable.

You can tear or strain any of the tissues that hold your knee together: Ligaments connect bones to each other; tendons connect muscle to bone. Irritated tendons from using them too much? That’s tendinitis.