Ultrasonics and Acousto-Optics for the Nondestructive Testing of Complex Materials Nondestructive testing has become an essential tool for failure prevention, conditionbased maintenance, structural health monitoring, as well as for quality control during production processes. Ultrasound and Acousto-Optics are applied to examine the mechanical properties of isotropic and anisotropic materials, and also to detect material defects. Fiber reinforced composites are ubiquitous in many industries, yet the interaction of ultrasound with such materials is particularly challenging because of effects such as surface roughness, the fact that materials in principle may be triclinic instead of orthotropic, the presence of a pre-stress, piezoelectric effects, the existence of a coating, the finite dimensions of material parts, etc. Some important recent results will be presented. The presentation will focus on the interaction of ultrasound with multi-layered fiber reinforced composites and crystals, the propagation of ultrasound in piezoelectric materials that are subject to a pre-stress, and diffraction phenomena on 1D and 2D corrugated surfaces