The chapter deals with two recently proposed characterization techniques of microwave transistors oriented to high-frequency power amplifier (PA) design. In particular, the nonlinear embedding and de-embedding design techniques are detailed, along with evidence of their advantages with respect to conventional design approaches in terms of power and frequency handling capability. The discussion also details the differences between the two techniques; despite the fact that they share the same theoretical basis, the techniques suffer from different critical facets. Finally, with the aim of guiding the reader towards full comprehension of the topic, different experimental examples are provided for transistor characterization and PA design.