Publisher Summary Data sheets are often the sole source of information about the capability and characteristics of a product. This chapter reviews the RF transistor and amplifier module parameters from maximum ratings to functional characteristics. The section is divided into five basic parts. The first part gives an overview of the D.C. specifications, power transistors, low power transistors, power modules, and linear modules. Comments are made in the chapter about critical specifications, value determination, and their significance. It presents a brief description of the procedures used to obtain impedance data and thermal data along with the elaboration of the importance of test circuits. Background information is provided in the chapter to help the reader understand low noise considerations and linearity requirements. World-class data sheets in the next few years will tend to provide more and more information about the characteristics of the RF device; information that will be directly applicable to the engineer in using the device.