Photoacoustic tomography (PAT) has attracted a growing interest in the field of medical imaging in the recent decade. PAT incorporates advantages attributed to both light and sound, including a high contrast in discriminating different biological targets provided by light, and a high spatial resolution ensured by sound. This hybrid nature leads to two distinct inverse problems: the optical inverse problem, which addresses the propagation of the excitation light in tissue, and the acoustic inverse problem, which deals with the propagation of the generated acoustic waves. The two inverse problems are based on different physical fundamentals, so they are often addressed by different types of equations independently. This seminar is focused on the acoustic aspects of PAT.