Photoacoustic tomography (PAT) is an emerging imaging technology that leverages the photoacoustic effect to create multiscale, multi-contrast images of biological structures from organelles to organs. By converting light absorption into ultrasonic waves, PAT overcomes the high scattering of optical photons in biological tissues, achieving high spatial resolution and depth. The technology provides anatomical, functional, metabolic, molecular, and genetic contrasts, making it versatile for various biological and clinical applications. PAT has been rapidly developed over the last decade, with applications in vascular biology, oncology, neurology, ophthalmology, dermatology, gastroenterology, and cardiology. The article reviews the fundamentals and major implementations of PAT, including focused-scanning photoacoustic microscopy (PAM), photoacoustic computed tomography (PACT), and photoacoustic endoscopy (PAE). It highlights the scalability of PAT across different length scales, from organelles to organs, and demonstrates its ability to image a wide range of endogenous and exogenous contrast agents. The authors also discuss the potential of PAT in metabolic imaging and its impact on biomedicine, while acknowledging the technical challenges that need to be addressed for broader clinical application.Photoacoustic tomography (PAT) is an emerging imaging technology that leverages the photoacoustic effect to create multiscale, multi-contrast images of biological structures from organelles to organs. By converting light absorption into ultrasonic waves, PAT overcomes the high scattering of optical photons in biological tissues, achieving high spatial resolution and depth. The technology provides anatomical, functional, metabolic, molecular, and genetic contrasts, making it versatile for various biological and clinical applications. PAT has been rapidly developed over the last decade, with applications in vascular biology, oncology, neurology, ophthalmology, dermatology, gastroenterology, and cardiology. The article reviews the fundamentals and major implementations of PAT, including focused-scanning photoacoustic microscopy (PAM), photoacoustic computed tomography (PACT), and photoacoustic endoscopy (PAE). It highlights the scalability of PAT across different length scales, from organelles to organs, and demonstrates its ability to image a wide range of endogenous and exogenous contrast agents. The authors also discuss the potential of PAT in metabolic imaging and its impact on biomedicine, while acknowledging the technical challenges that need to be addressed for broader clinical application.