This chapter introduces some of the basic concepts of physics necessary to appreciate instrumental methods of chemical analysis. We first consider the behavior of waves from a general point of view, and then specifically relating to electromagnetic (EM) waves. We then present one of the most astonishing results of quantum mechanics – the fact that light can be considered either as a wave, or as a stream of particles (photons), depending on the way in which we choose to observe it. The next section looks at one of the important consequences of quantized electronic orbitals in atoms and molecules – the fact that they absorb and emit energy (including visible light) at specific wavelengths, rather than as a continuous spectrum, and also that the pattern of these wavelengths is unique to each element. Moreover, using an equation (Beer's law) for the absorption of electromagnetic radiation by matter, we show that it is possible to quantify the amount of each element present in a sample. This is the basis of many forms of chemical analysis. By considering the range of energies covered by the different wavelengths within the electromagnetic spectrum (from high energy, short wavelength γ-rays to low energy, long wavelength radio waves), we can see that different energy levels within the atom are probed by different types of radiation.