Photoacoustic imaging is a hybrid modality that combines the molecular sensitivity of optical absorption with the spatial resolution of ultrasound. It works by delivering short optical pulses into tissue. The absorbed light causes thermoelastic expansion, which generates ultrasonic waves that can be detected and reconstructed into images. This chapter introduces the physical principles of photoacoustic imaging, describes typical system architectures that have been developed for biomedical applications, and situates the modality within the broader landscape of medical imaging. Particular emphasis is placed on the limitations of conventional laser-based systems, including high cost, system complexity, and restricted accessibility. To address these barriers, we highlight emerging strategies for building affordable, scalable solutions that rely on light-emitting diodes, laser diodes, and cost-effective acoustic detection. The chapter concludes by outlining a roadmap for subsequent chapters.