This chapter discusses the basic ideas of the Standard Model of particle physics, in which quarks and leptons interact via the exchange of gauge field quanta. From today’s perspective, the crucial thing about electromagnetism is that it is a theory in which the dynamics is intimately related to a symmetry principle. A distinction that is vital to the understanding of all these interactions is that between a global invariance and a local invariance. In a global invariance the same transformation is carried out at all space–time points: it has an ‘everywhere simultaneously’ character. In a local invariance different transformations are carried out at different individual space–time points. The Schrödinger equation is non-relativistic, but the Maxwell equations are of course fully relativistic. One might therefore suspect that the prescriptions discovered here are actually true relativistically as well, and this is indeed the case.