Photoacoustic imaging systems can be categorized as being computed tomography or scanning-based. In the former implementation, known as photoacoustic computed tomography (PACT), an image reconstruction algorithm is employed to produce a two- or three-dimensional estimate of the absorbed optical energy density, or related quantity, from knowledge of the recorded ultrasonic waves. A fixed transducer array can be employed to record these data quickly, thereby eliminating the need for any mechanical scanning. Dedicated PACT imaging systems are being currently investigated by several groups for small animal imaging applications and human breast cancer detection and management. On the other hand, in scanning-based photoacoustic imaging methods, the photoacoustic wavefields are detected by use of a focused ultrasonic transducer to form a depth-resolved one-dimensional image directly. By scanning the transducer over a two-dimensional surface, a three-dimensional image can be obtained. While scanning can result in increased data-acquisition times as compared to PACT systems that employ fixed arrays, it can be employed with high-bandwidth transducers and small scanning step sizes to achieve micron-scale spatial resolution, or better, which facilitates photoacoustic microscopy applications. Both PACT and scanning-based methods can be implemented in spectroscopic mode and are able to provide functional imaging of physiological parameters. Such parameters include the concentration and oxygen saturation of hemoglobin and molecular imaging of biomarkers and gene expression products.