ACL is Anterior Cruciate Ligament. It is one of the most important ligaments in knees. This ligament is situated behind the kneecap. This ligament helps in giving stabilization in the knees. Besides ACL, there are other 3 ligaments in knees. They are PCL (posterior cruciate ligament), LCL (lateral collateral ligament) and MCL (medial collateral ligament)..