This paper summarizes the characteristics of nonlinear ultrasonic guided waves and demonstrates the use of the nondispersive fundamental shear horizontal wave mode to generate a third harmonic that is sensitive to plastic strain and fatigue. Experimental results for an aluminum alloy plate are presented within the context of the current understanding of nonlinear guided waves. Due to the sensitive nature of higher harmonic measurements, instrumentation nonlinearities are discussed. In two different sets of experiments, the third harmonic generation is shown to be dependent upon (i) the plastic strain and (ii) the number of fatigue cycles. Thus, higher harmonic generation can provide material state awareness throughout the service life, especially before a macroscale crack initiates.