Both the clinical examination and the examination under anesthesia for anterior cruciate ligament (ACL) instability involve a large number of false-positive and false-negative diagnoses. The greatest uncertainty is attached to the examination of the anterior cruciate ligament. The diagnostic specificity and the diagnostic sensitivity of the clinical examination tests of the anterior cruciate ligament are 0.75 and 0.62, respectively. The majority of orthopedists are very skilled in the diagnosis of ACL tears. However, acute ACL tear and partial lesion are perhaps the most underdiagnosed orthopedic condition. Sensitivity rate of 75% has been reported using MRI. The MRI was very sensitive for detecting ACL injuries, but it was much less specific for differentiating the total lesion from the partial lesion of ACL and a simple laxity of ACL. Classic active, passive, and dynamic clinical tests and traditional MRI have been supplemented by an additional functional clinical test, the Lelli’s Test, and dynamic MRI, which have helped us to more reliably diagnose total and partial injuries of the ACL and to understand more precisely when the ACL, although appearing “healthy,” is functioning well or not, meaning whether it is functionally valid or not.