This chapter explores the characteristics and applications of light sources in photoelectric detection systems, focusing on their impact on performance, selection, and optimization. It categorizes light sources into natural and artificial types, detailing their roles in detecting temperature, pressure, and velocity. Key parameters (i.e. brightness, spectral distribution, stability, lifespan) and various light sources (i.e. thermal radiation sources, gas discharge lamps) are discussed, alongside applications in biomedical imaging, security monitoring, and communication systems. The chapter discusses spectral properties and emission mechanisms to provide insights into radiation energy distribution, and also explores advancements in light source technology. It concludes by highlighting the evolution of LED and laser technologies, particularly in fluorescence detection, medical diagnostics, and sensor calibration.