Thermal wave imaging of defects in opaque solids is carried out by focusing the periodically modulated intensity of a heat source (conventionally a laser, electron beam, or ion beam) at the surface of the solid. Solution of the heat equation shows that the ac temperature in the solid is wave-like (thermal waves), and critically damped. These waves can be used to probe the thermal properties of the subsurface of the solid, and their reflections from discontinuities in thermal impedance can be used to image subsurface defects such as inclusions, voids, cracks, and delaminations. Various techniques have been developed to detect the resulting ac temperature variations at the sample surface, including photoacoustic (gas-cell) detection, thermoacoustic (piezoelectric transducer) detection, optical probe beams, and ac infrared detection. Thermal wave imaging is particularly useful to probe subsurfaces from depths of about 1 micron to 300 microns, with a maximum depth of about 2 mm.