Photoacoustic tomography (PAT) refers to imaging that is based on the photoacoustic effect. Although the photoacoustic effect as a physical phenomenon was first reported on by Alexander Graham Bell in 1880 , PAT as an imaging technology was developed only after the advent of ultrasonic transducers, computers, and lasers [2–31]. A review on biomedical photoacoustics is available . The motivation for PAT is to combine optical-absorption contrast with ultrasonic spatial resolution for deep imaging in the optical quasi-diffusive or diffusive regime. In PAT, the tissue is irradiated by usually a short-pulsed laser beam to achieve a thermal and acoustic impulse response (Fig. 19.1). Locally absorbed light is converted into heat, which is further converted to a pressure rise via thermo-elastic expansion. The initial pressure rise – determined by the local optical absorption coefficient (μ_â), fluence (ψ) and other thermal and mechanical properties – propagates as an ultrasonic wave, which is referred to as a photoacoustic wave.