While many articles have been written on microwave devices, a great majority of them are prepared for specialists dealing in specific aspects of microwave engineering. At the same time, material at a fundamental level in tutorial form is extremely limited, especially for stu- dents who need to acquire basic knowledge in the field. Individuals seeking to gain a prelim- inary understanding of microwave circuits are usually relegated with little success to the end- less search from one reference source to another. For non-experts, sequential derivations of basic relations are rarely available and extremely difficult to locate. The purpose of this volume is to collect in one place the essential fundamental principles for a group of microwave devices. The chosen devices are those which form the basic modules found in practical microwave systems. Thus, these devices provide the crucial build- ing blocks in common microwave systems, and their inherent characteristics are also the basis of some of the fundamental concepts in more complex devices. The material is presented in a continuous, self-contained manner. With the appropriate background, readers should be able to follow and understand the contents without the need for additional references.