The objective of this investigation was to produce experimental verification of the tearing instability theory proposed by Paris and co-workers [10].³ This theory states that ductile crack extension will occur in an unstable fashion whenever the applied tearing force is greater than the material tearing resistance. In this investigation a series of compact specimens of aluminum, titanium, and steel alloys was tested in a variably compliant test machine to generate a range of applied tearing force. The material tearing resistance was measured from the J_I-R curves of the stable specimens and compared with the applied tearing force necessary to generate ductile tearing instability in each material. The Paris theory was found to accurately predict the onset of gross instability behavior. Some limited instability behavior was found, however, at values of tearing force less than the average material tearing resistance obtained from an unloading compliance J_I-R curve test. Limited instability behavior was characterized by repeated short steps of rapid but ductile crack extension, separated by regions of slow stable tearing.