The anterior cruciate ligament (ACL) is one of the most commonly injured knee ligaments, with nearly 200,000 injuries annually in the US, with ~100,000 requiring reconstruction. The ACL prevents anterior translation of the tibia relative to the femur and is a secondary restraint to tibial and varus/valgus rotation. Accurately diagnosing ACL injuries in the ED after an acute knee injury remains difficult. The diagnosis is complicated because clinical tests are performed on an acutely injured knee, which is likely swollen and painful, leading to muscle contractures and patient apprehension. As Emergency Medicine physicians, we must have some confidence in suspicion of an ACL tear because not all patients can or should be referred for prompt orthopedic follow-up.