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Microwave circuits generally include a number of junctions and discontinuities at the interconnection of the various circuit limits and the resulting parasitic reactances can have a major effect on the circuit operation. The designer of waveguide and coaxial circuits has had available, for many years, a wealth of information on the parasitic reactances associated with the circuit elements and discontinuities, such as bends, T-junctions, transitions. Many elements can be evaluated theoretically and very accurate measurement techniques have also been devised in both media. Furthermore, small adjustments to the final circuits may easily be made using tuning screws, movable short-circuits, etc. In microwave integrated circuits the theoretical evaluation of circuit elements and discontinuities is difficult, experimental measurement is of limited accuracy and adjustment of the final printed circuit not easy to achieve. This paper will review the available theoretical and experimental methods for evaluating such discontinuities and present the available range of results. The equivalent capacitances can be obtained theoretically using fairly well known techniques but the inductance calculations are more difficult, although some progress has been made in this area recently. Theoretical data are important as accurate experimental measurements require considerable effort and there are a large range of configurations in use. To illustrate the effect of discontinuities on practical design the case of a microwave f.e.t. amplifier will be taken. Significant design errors can be shown to occur if the discontinuities are not allowed for. However, by using the data we now have available on the parasitics, fairly simple modifications can be introduced in the initial design to enable the desired performance to be more directly obtained.