Whether or not a partial ACL tear should be treated conservatively or by operative reconstruction remains controversial. In case of instability during stair climbing or direction changes we suggest an operative treatment. Exact clinical examination of the knee is mandatory to identify minor laxities in translation or rotation. MRI imaging is not highly significant for the diagnosis of a partial ACL tear; however, it can rule out concomitant injuries at the cartilage or meniscus. The best test in the diagnosis of a partial ACL tear is meticulous probing of the ACL during diagnostic arthroscopy at different knee flexion angles with anterior tibial translation loads. If sufficient parts of the ligament remain, an augmentation with a hamstring tendon autograft can be accomplished with preservation of the remaining intact ligament. An additional indication for partial ACL repair is to correct an intact, but previously misplaced “high-noon” ACL reconstruction with a more oblique bundle (anatomical) to improve rotational stability. This chapter deals with some key points in the diagnosis and therapy of the partial ACL tear. The authors’ preferred surgical technique for augmentation is described in detail.