A general analysis of circular wire loops with arbitrary electromagnetic excitation and admittance loading is given. The analysis is made in terms of generalised network parameters and is equivalent to previous analyses of particular problems. Some important special cases of the general theory are circular-loop antennas, loaded-loop antennas, circular-loop scatterers and loaded circular-loop scatterers. A number of new computations of radiation and scattering patterns are given to illustrate the general behaviour of wire loops.