Photoacoustic tomography (PAT) is a promising imaging technique that combines optical and ultrasonic technologies to create multiscale, multi-contrast images of biological structures ranging from organelles to organs. By using the photoacoustic effect, PAT converts optical absorption into ultrasonic waves, which are then detected to form images. This method overcomes the limitations of optical imaging in biological tissues, such as scattering, by leveraging the low acoustic scattering of tissue. PAT provides high-resolution images with a depth-to-resolution ratio of approximately 200, enabling imaging up to 7 cm in depth. It offers anatomical, functional, metabolic, molecular, and genetic contrasts, making it valuable for both biological and clinical studies. PAT has three major implementations: focused-scanning photoacoustic microscopy (PAM), photoacoustic computed tomography (PACT), and photoacoustic endoscopy (PAE). PAM provides high-resolution images at the subcellular level, while PACT enables both microscopic and macroscopic imaging. PAE is used for endoscopic imaging of internal organs. PAT's ability to image at multiple scales makes it suitable for studying organelles, cells, tissues, and organs. It can detect various biological processes, including blood flow, temperature changes, and metabolic activity, using endogenous and exogenous contrast agents. PAT has been applied in various fields, including vascular biology, oncology, neurology, ophthalmology, dermatology, gastroenterology, and cardiology. It offers advantages such as high sensitivity, background-free detection, and the ability to image without fluorescent labeling. PAT is also capable of imaging deep tissues with high resolution, making it useful for in vivo imaging. However, challenges remain in achieving high-speed, multi-contrast imaging and improving the resolution and depth of imaging in clinical settings. The scalability of PAT allows it to bridge the gap between microscopic and macroscopic imaging, providing a unique opportunity to study biological systems at multiple length scales. PAT has the potential to revolutionize biomedical imaging by enabling non-invasive, high-resolution imaging of metabolic processes, cancer detection, and other biological functions. Despite its promise, PAT still faces technical challenges that need to be addressed to maximize its impact in biomedicine.Photoacoustic tomography (PAT) is a promising imaging technique that combines optical and ultrasonic technologies to create multiscale, multi-contrast images of biological structures ranging from organelles to organs. By using the photoacoustic effect, PAT converts optical absorption into ultrasonic waves, which are then detected to form images. This method overcomes the limitations of optical imaging in biological tissues, such as scattering, by leveraging the low acoustic scattering of tissue. PAT provides high-resolution images with a depth-to-resolution ratio of approximately 200, enabling imaging up to 7 cm in depth. It offers anatomical, functional, metabolic, molecular, and genetic contrasts, making it valuable for both biological and clinical studies. PAT has three major implementations: focused-scanning photoacoustic microscopy (PAM), photoacoustic computed tomography (PACT), and photoacoustic endoscopy (PAE). PAM provides high-resolution images at the subcellular level, while PACT enables both microscopic and macroscopic imaging. PAE is used for endoscopic imaging of internal organs. PAT's ability to image at multiple scales makes it suitable for studying organelles, cells, tissues, and organs. It can detect various biological processes, including blood flow, temperature changes, and metabolic activity, using endogenous and exogenous contrast agents. PAT has been applied in various fields, including vascular biology, oncology, neurology, ophthalmology, dermatology, gastroenterology, and cardiology. It offers advantages such as high sensitivity, background-free detection, and the ability to image without fluorescent labeling. PAT is also capable of imaging deep tissues with high resolution, making it useful for in vivo imaging. However, challenges remain in achieving high-speed, multi-contrast imaging and improving the resolution and depth of imaging in clinical settings. The scalability of PAT allows it to bridge the gap between microscopic and macroscopic imaging, providing a unique opportunity to study biological systems at multiple length scales. PAT has the potential to revolutionize biomedical imaging by enabling non-invasive, high-resolution imaging of metabolic processes, cancer detection, and other biological functions. Despite its promise, PAT still faces technical challenges that need to be addressed to maximize its impact in biomedicine.