This chapter provides a physical background to the following ones. It describes the particle and wave properties of light, and the diffraction, polarization, refraction, reflection, and absorption of light, statistics of photon emission and absorption. Planck's law of heat radiation is described in various mathematical and graphical ways. One section is devoted to a simplified description of the propagation of light in absorbing and scattering media. The final sections are devoted to interactions between light and matter: spectra of and energy levels in atoms and molecules, the relation between absorption and emission spectra, the molecular geometry between absorption and emission, and the transfer of electronic energy, including the Förster mechanism, triplet states, and the photobiologically important properties of the dioxygen molecule.