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This paper is intended to be tutorial in nature and its purpose is the description of the effects of magnetically saturated ferrites in microwave structures from the more general approach of microwave circuit theory. This is done via the scattering matrix rather than the impedance matrix. Since most of the important properties of such circuits depend on the fact that the circuit behaves differently when the direction of propagation is reversed, the scattering matrix seems to be a more natural mode of description than the impedance matrix from which these effects are not immediately evident. The properties of the scattering of microwave junctions containing magnetically saturated media are discussed. It is shown that except for nonreciprocity, the scattering matrix of such junctions satisfy the same requirements as that of junctions containing ordinary scalar media. The general scattering matrix of a lossless two port device is given. The circulator is discussed and a particular microwave circuit, of which several proposed circulators are a special case, is treated. Conditions on the elements of this circulator, in order that it be a perfect circulator, are derived. Finally deviations from the ideal circulator conditions are considered.