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Photoacoustic tomography (PAT), also known as thermoacoustic or optoacoustic tomography, is a rapidly emerging imaging technique that holds great promise for biomedical imaging. PAT is a hybrid imaging technique, and can be viewed either as an ultrasound mediated electromagnetic modality or an ultrasound modality that exploits electromagnetic-enhanced image contrast. In this chapter, we provide a review of the underlying imaging physics and contrast mechanisms in PAT. Additionally, the imaging models that relate the measured photoacoustic wavefields to the sought-after optical absorption distribution are described in their continuous and discrete forms. The basic principles of image reconstruction from discrete measurement data are presented, which includes a review of methods for modeling the measurement system response.