This paper deals with monitoring impacts on aluminum and composite aerospace panels. The specific problems addressed are (1) the identification of the impact forces (force magnitude time history) and (2) the discrimination of "damaging impacts" from "non-damaging impacts." Ultrasonic guided waves generated by the impacts are used as the sensing mechanism. Flexible Macro-Fiber Composite (MFC) patches are used as the ultrasonic receivers. The impact force identification method is based on an optimization routine which minimizes the difference between the experimental impact waves and the numerical impact waves calculated by a Semi-Analytical Finite Element (SAFE) forced analysis. The differentiation of "damaging impacts" vs. "non-damaging impacts" is based on a frequency analysis of the impact waves. These techniques are demonstrated through an extensive experimental testing program involving the following six specimens: an aluminum panel, a quasi-isotropic CFRP composite panel, a highly anisotropic CFRP composite panel, a stiffened aluminum panel, a stiffened quasi-isotropic CFRP composite panel, and a stiffened anisotropic CFRP composite panel. These panels were subjected to low-velocity hammer impacts and to high-velocity gas-gun impacts by ice projectiles at speeds up to 170 m/sec using UCSD's gas-gun test facility. In all of these experiments, the techniques summarized above gave excellent results for both impact force identification and impact force discrimination.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.