Crack growth that results in a sudden release of elastic energy can be detected by use of a piezoelectric crystal that will convert the elastic stress wave into an electrical signal. Data on two widely different alloys, 7075-T6 aluminum and HY-80 steel, are presented in this paper. The objective was to determine (1) if, in these materials there is some recognizable, reproducible stress-wave characteristic(s) which predicts the onset of crack instability under plane-strain and plane-stress conditions; (2) if such characteristic varies from one material to another, and (3) if such characteristic is affected by specimen configuration. Single-edge-notch tensile tests involving a range of widths, thicknesses and crack depths all showed essentially the same stress-wave behavior. Plane-stress instability was marked in both materials by a number of large amplitude stress waves. In 7075-T6, KIC was marked by an order-of-magnitude increase in stresswave amplitude; whereas, in HY-80, large amounts of crack growth occurred at stress intensity levels approaching the yield stress with stress-wave emission so small as to be barely detectable at the sensitivity level used. Fracture in 7075-T6 was normally in a flat, plane-strain mode; whereas, in HY-80, it was almost entirely in oblique shear by microvoid coalescence.