This chapter provides a basic review of electric and magnetic fields and the relations between these fields. Maxwell's equations defining these relations have been known for a long time, however, the solutions to these equations are often complex, as the biological materials can be inhomogeneous, nonlinear, time varying and anisotropic. A number of approximations which depend on the ratio of the wavelength of the electromagnetic waves to the dimensions of the body being exposed are presented that simplify the calculations of the field strengths at a given location. In the case of time-changing fields, different terms in Maxwell's equations dominate when distance from the source to the object is large or small compared to the size of the source; these are the far-field and near-field situations. Introduction template: The chapter also presents an overview on the key concepts discussed in this book.