I will be describing an acoustic emission program at the National Bureau of Standards sponsored jointly by NBS and the Electric Power Research Institute. This is part of a larger comprehensive NDE program at NBS. The acoustic emission program uses the spectral analysis approach for the characterization of moving defects and, as I will point out, includes the development of an acoustic emission transducer calibration facility as well as the experimental study of crack motion in glass and pressure vessel steels. These require a quantitative theory which serves to guide the experimental design as well as to interpret the results.