The article provides an overview of photoacoustic imaging (PAI), a technique that combines high-resolution ultrasonic imaging with high contrast due to light or radio-frequency (RF) absorption. PAI is particularly useful for imaging animal or human organs, such as the breast and brain, with high contrast and spatial resolution. The authors discuss various imaging techniques, including depth profiling in layered media, scanning tomography with focused ultrasonic transducers, image forming with acoustic lenses, and computed tomography (CT) with unfocused transducers. Special emphasis is placed on CT, including reconstruction algorithms, spatial resolution, and recent experiments. Promising biomedical applications are discussed, such as tomographic imaging of the skin and other superficial organs using laser-induced PAI, breast cancer detection using near-infrared light or RF-induced PAI, and small animal imaging using laser-induced PAI. The article also covers the physical principles of PAI, the properties of biological tissues in the optical and RF regions, safety considerations, and the generation and detection of PA signals.The article provides an overview of photoacoustic imaging (PAI), a technique that combines high-resolution ultrasonic imaging with high contrast due to light or radio-frequency (RF) absorption. PAI is particularly useful for imaging animal or human organs, such as the breast and brain, with high contrast and spatial resolution. The authors discuss various imaging techniques, including depth profiling in layered media, scanning tomography with focused ultrasonic transducers, image forming with acoustic lenses, and computed tomography (CT) with unfocused transducers. Special emphasis is placed on CT, including reconstruction algorithms, spatial resolution, and recent experiments. Promising biomedical applications are discussed, such as tomographic imaging of the skin and other superficial organs using laser-induced PAI, breast cancer detection using near-infrared light or RF-induced PAI, and small animal imaging using laser-induced PAI. The article also covers the physical principles of PAI, the properties of biological tissues in the optical and RF regions, safety considerations, and the generation and detection of PA signals.