Photoacoustic Imaging (PAI), also known as optoacoustic imaging, is a promising hybrid technology that integrates the advantages of optical and ultrasound imaging to achieve high-resolution, label-free imaging of deep biological tissues. Utilizing the PA effect, PAI converts absorbed optical energy into ultrasound signals, enabling detailed structural and functional assessment of tissues with minimal invasiveness. This chapter explores the fundamental principles of PAI, focusing on its signal generation and detection mechanisms, the roles of endogenous and exogenous contrast agents, and various imaging methodologies, including PA spectroscopy, microscopy, remote sensing, and tomography. Additionally, it examines advanced image reconstruction techniques critical for enhancing the quality of PA images, emphasizing the need for practical algorithms due to the complexities of acoustic signal propagation within heterogeneous biological tissues. As PAI technology advances, integrating artificial intelligence into imaging analyses and employing novel detectors and light sources will undoubtedly enhance its applications and diagnostic capabilities. The discussion culminates with insights into ongoing challenges and future directions, particularly concerning developing portable and cost-effective systems that can augment the clinical utility of PAI in diverse healthcare settings. Overall, this chapter provides a comprehensive overview of the fundamentals, technological innovations, and prospective advancements in the field of PAI, highlighting its transformative potential in biomedical research and clinical practice.