This lecture introduces attendees to the GaN-transistor, its properties, various structures, including the latest GaN power amplifier (PA) design techniques. The properties of GaN will be presented showing the advantage of these devices over GaAs and Si. GaN HEMT transistors will be shown delineating the various geometries, semiconductor processes and structures with associated performance. Guidelines for reliable operation will be presented considering device junction temperature including thermal management techniques. The nonlinear models of GaN HEMT devices necessary for the CAD of PAs will be presented. Design considerations for both constant amplitude envelope signals as well as the non-constant amplitude envelope signals will be presented. Step-by-step design procedures will be shown for various GaN PA examples including different classes of operation as well as the popular Doherty PA.