This book embarks on an exploration of the interplay between excitons, which are the fundamental excitations in semiconductors, and photons, which are the electromagnetic quanta, providing fresh insights into light-matter interaction. Covering the basic principles of semiconductor electronic structures in a detailed yet accessible way, this book provides a pedagogical introduction to the various Coulomb interactions between electrons and holes, which are the two semiconductor key particles, as well as the mathematical tools to study their many-body effects. It untangles the tricky excitonic physics that develops from the interplay between Coulomb and electron-photon interactions, which differentiates bright from dark excitons or leads to the bosonic modes known as exciton-polaritons. A unique re-examination of well-established concepts, this book provides original presentations of fundamental questions, such as the polariton vacuum or the deceptively simple exciton creation through photon absorption. Whether you are a student eager to learn from step-by-step derivations or an experienced physicist seeking to refine your knowledge, this book will serve as an invaluable guide to mastering the microscopic physics of semiconductors, an important material in modern technologies.