This chapter introduces the principles of electromagnetic theory which are essential to an understanding of radiowave propagation. Where possible, a practical engineering approach has been adopted. However, the material has been structured to provide those with a more mathematical background with a satisfactory account of the subject, together with references to further reading material. Initially, propagation between two antennas in free space is considered by means of power flux concepts. This enables important ideas such as free-space path loss, antenna gain and effective aperture to be introduced. Maxwell's equations are presented. and plane-wave solutions are derived as a means of introducing polarisation, wave impedance and electromagnetic power flow. Finally, radiation from a current distribution is examined and illustrated by deriving the fields of a dipole antenna.