Why do my knees crack when I squat?

Usually it is harmless joint noise, and squatting is simply when your knees move through their biggest range, so the sounds get loudest. The cracks come from gas bubbles shifting in the joint fluid as pressure changes, and from tendons and ligaments snapping over bone as the knee bends deep. NHS guidance is blunt about the dividing line: painless clicking is normal, while a knee that locks, gives way, or painfully clicks needs checking. Noise alone, without pain, swelling, or a feeling of catching, is not a sign of damage and does not mean you are wearing your knees out, and it is not a reason to avoid squats. The versions that matter: cracking that started right after a twist or fall, noise with swelling that appears hours later, pain every time you bend, a knee that catches or locks mid-movement, or one that buckles. Those point to cartilage, meniscus, or ligament problems and deserve a GP or physio assessment. If the noise bothers you, strengthening the quads, hips, and glutes often quiets knees down by improving how the kneecap tracks, and warming up before deep bending helps too.