The evolution of thermal wave imaging and materials characterization is traced from its origins during the time of the First International Workshop on Photoacoustics and Photothermal Phenomena in Ames, Iowa in 1979 to the present, and with an eye to the future. In the early days, the heat sources consisted of amplitude-modulated lasers, focused to a spot, and step-scanned across the surface of the object under evaluation. A variety of lock-in detection schemes were used, including microphones in gas cells (photoacoustics), laser optical probes (the mirage effect), photothermal defection, thermoreflectance, and infrared (IR) detection. With the commercial availability of IR cameras, rapid and wide-area synchronous imaging became possible, both in the frequency domain (lock-in imaging), and the time-domain (box-car imaging). Recently, the photoacoustic technique has been “flipped,” with a pulse of sound being used as the energy source, and with an IR camera monitoring the subsequent photons emitted in the v...