The application of antimicrobial photodynamic therapy (aPDT) takes a precise understanding of light physics and dosimetry to maximize its efficacy while minimizing unintended effects. This chapter explores the fundamental principles of light, focusing on its wavelength, energy, and interactions with biological tissues, which underpin the activation of photosensitizers (PSs) and the generation of reactive oxygen species (ROS). Various light sources, including lasers, light-emitting diodes (LEDs), and broadband lamps, are examined in terms of their spectral characteristics, power output, and suitability for clinical applications. The chapter highlights critical dosimetry parameters such as optical energy, power, and exposure time alongside their influence on microbial inactivation and host tissue response. This chapter provides a comprehensive foundation for understanding the interplay of physics and dosimetry in aPDT, offering insights into current practices and innovations shaping its clinical translation.