Book excerpt: The most promising class of materials poised to take over the future of microwave power transistors are wide band gap semiconductors among which GaN is most exciting. A high electron mobility transistor (HEMT) made of an AlGaN/GaN heterostructure can take advantage of a sheet charge layer known as a two-dimensional electron gas (2DEG) to achieve superior properties compared to other materials such as Si, SiC and GaAs. Unfortunately AlGaN/GaN HEMTs are plagued by unknown reliability and there is no clear mechanism which describes how these devices degrade. Degradation of AlGaN/GaN HEMTs is a result of multiple mechanisms, and from a materials perspective it is most advantageous to study the physical degradation and microstructural changes of these electronic devices. During high voltage and high frequency operation AlGaN/GaN HEMTs experience irreversible degradation in their electronic properties, and this degradation has been attributed to physical degradation of the device1-7. By performing high resolution transmission electron microscope (HRTEM) imaging and taking advantage of the phase information within a bright field TEM image by performing Geometric Phase Analysis, a higher resolution and greater quantitative understanding of the degradation of AlGaN/GaN HEMTs can be achieved. Establishing the ability to reproducibly perform HRTEM and quantitative analysis on AlGaN/GaN HEMTs of varying amount of bias is only the first step on the road toward to the ultimate goal of performing in-situ TEM on these devices. Much of the ground work for in-situ studies has been laid out and key issues for moving forward are discussed.