In Chapter 1, some of the topics in the conventional circuit theory are reviewed, which have particular importance in the theory of microwave circuits. There include the theory of transmission line, bilinear transformations, and power waves. Chapter 2 gives a review of vector analysis and the fundamental properties of electromagnetic fields to facilitate our later study. In Chapter 3, waveguides are discussed for the first time. Here, the eigenvalue problem is studies in detail including the completeness of eigenfunctions. Without the discussion of the completeness, the theory is only partially correct since no answer is given to certain fundamental questions such as why an exponential variation with distance is assumed for each mode and no other pos...