Instrumented measurement of knee motion can assist the clinician in the diagnosis of an anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) disruption. It provides documentation of the amount of pathologic laxity, which can assist in determining which patients may benefit from an ACL reconstruction and provide a general prognosticator for a satisfactory result. Measurements can also indicate the success of an ACL reconstruction in returning a patient's knee to normal joint kinetics. Any surgeon interested in cruciate ligament research or publishing in scientific journals should have arthrometric data to support his or her conclusions. However, it is still important to do a thorough physical examination and correlate these findings with arthrometric data. In the hands of experienced examiners, 90% to 98% of acute ACL tears can be accurately diagnosed using arthrometers. Surprisingly, there is significant variability when comparing physical findings. Among less experienced examiners using arthrometers, intra-examiner variability remains low, but inter-examiner variability remains only fair.