Two new techniques have been developed for non-destructive detection of a wide spectrum of microstructural defects in crystalline materials: resonant ultrasound spectroscopy and non-linear acoustics.  Broad band resonant ultrasound spectroscopy coupled with “forward modeling” of microstructure enables detection of near-surface and sub-surface defects that arise during material processing through detailed analysis of shifts in different resonance modes. Nonlinear ultrasonic studies explore the generation of second and higher harmonics of the fundamental frequency due to distortion of sinusoidal ultrasonic waves as they propagate through a nonlinear or anharmonic solid. We have observed for the first time that the ultrasonic nonlinearity during fatigue cycling increases with evolution of dislocation structure, crack initiation and growth of damage.