This chapter makes a brief introduction of the GaN-HEMT technological process development. Based on this technology, it is established a design procedure for broadband high power amplifiers. The design is focused on the synthesis of the matching and stabilization networks of a two-stage amplifier. It is highlighted the need for nonlinear stability analysis to avoid parametric and odd-mode oscillation. Thermal characterization is also critical due to the high power dissipated in high power GaN devices. Finally, we present the analysis of results of two broadband HPA demonstrators.