This article presents a comprehensive overview of kilowatt-level RF power transistors and transistor amplifiers capable of 100 kW or more output power. Discussion begins with transistor material parameters, thermal conductivity, and the coefficient of thermal expansion (CTE) of common materials used in RF power transistors to provide a basis to understand the challenges and limitations of these devices in high power RF applications. A two-stage transistor characterization process is described that emphasizes different considerations at the wafer-level and the package-level. Finally, practical considerations are made for power combining at the circuit level, and at a scale of tens or even hundreds of power amplifiers with radial combining and e-plane waveguide combiner approaches.