Historically, electromagnetics and complex circuit modelling existed as separate disciplines, each with their own tools, models and even languages. More recently, however, the emergence of very high-speed digital circuits and pressure on the telecommunications market to move towards microwave and millimetre wave bands are increasing the need to find ways to combine the two fields. The consumer market demands low cost mass production devices operating at higher frequencies where the finite dimensions of the circuits with respect to wavelength can no longer be ignored. Similarly, integrated planar microwave circuits pose new modelling challenges, as neither conductors nor dielectrics can be considered as ideal at these frequencies and, consequently, most techniques and formulas developed over the past twenty years for dealing with ideal thin conductors no longer model the physical reality. These challenges are the main subject of this book which investigates analytical techniques encompassing the linear modelling of passive and active - in particular FET - structures. This timely book was primarily conceived as a bridge between the mathematical abilities of the pure EM theorist and those of the FET circuit modeller. However the resulting text will be of equal benefit to researchers in microwave and millimetric components and as a textbook for specialised courses.