This chapter discusses the fundamentals of the interactions of photons and especially X-rays with matter to give an introduction to X-ray absorption spectroscopy (XAS) in its basic form. As such, this chapter uses standard electrodynamics and quantum mechanics, from which the key XAS processes can be defined and presented. Later chapters will develop the theory and applications to old and new experimental fields within the broad umbrella of XAS. Approximations are made in this overview which will be questioned or adapted in later advanced topics. This introduction is predominantly nonrelativistic in development, using only those relativistic terms that are necessary to explain the physical processes.