Explore the fascinating world of mesoscopic physics of photons in this comprehensive lecture, the first of a three-part series delivered by Eric Akkermans from Technion, Israel. Delve into the intricate concepts and principles that govern the behavior of light at the mesoscopic scale. Learn about the fundamental aspects of photon interactions and their implications in various physical systems. Gain insights into cutting-edge research and applications in the field of optics and quantum mechanics. Enhance your understanding of light-matter interactions and their relevance to modern physics and technology. This lecture is part of the School on Light and Cold Atoms organized by ICTP-SAIFR, providing a solid foundation for further exploration of this captivating subject.