The organization of this text is as follows: Chapter 1 presents a general explanation of why basic circuit theory breaks down as the operating frequency is increased to a level where the wavelength becomes comparable with the discrete circuit components. In Chapter 2 the transmission line theory is developed as a way to replace the low-frequency circuit models. Because of the voltage and current wave nature, Chapter 3 introduces the Smith Chart as a generic tool to deal with the impedance behavior on the basis of the reflection coefficient. Chapter 4 discusses two-port networks with their flow-chart representations and how they can be described on the basis of the so-called scattering parameters. These network models and their scattering parameter descriptions are utilized in Chapter 5 to develop passive RF filter configurations. Before covering active devices, Chapter 6 provides a review of some of the key semiconductor fundamentals, followed by their circuit models representation in Chapter 7. The impedance matching and biasing of bipolar and field effect transistors is taken up in Chapter 8 in an effort to eliminate potentially dangerous reflections and to provide optimal power flow. Chapter 9 focuses on a number of key high-frequency amplifier configurations and their design intricacies ranging from low noise to high power applications. Finally, Chapter 10 introduces the reader to nonlinear systems and their designs in oscillator and mixer circuits.