Dielectric structures, in particular periodic dielectric structures are treated. A general one-dimensional model is developed describing Bragg mirrors. Examples for photonic band gap materials in one, two and three dimensions are given. Different types of dielectric cavities and microscopic resonators including Fabry–Pérot and whispering gallery resonators are treated. Quantum electrodynamic physical effects from light matter coupling such as Purcell effect and strong coupling are treated.