When performed incorrectly, even the best exercises can be ineffective and dangerous. Always consult a fitness professional if you are unsure of your form.
Squats are a popular exercise because they are effective at strengthening a wide range of muscles in a functional way. However, squats are also one of the most poorly executed exercises. When squatting, make sure your knees do not come over your toes, your back is straight and chest up and your thighs are parallel to the ground.