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In this article, we will introduce you to measurements for power transistor characterization: why they matter, why they are such a complicated, highly specialized field, and where we think the technology of power transistor characterization is headed. The characterization of microwave power transistors is an important and emerging field with many interesting engineering challenges. One can basically distinguish two areas: model extraction measurements and model validation measurements. To make things simple, isothermal pulsed-bias pulsed S-parameter measurements are typically used for model extraction purposes and load-pull measurements are typically used for model validation purposes. Both areas are rapidly evolving in order to keep track of new power transistor technology. The main issue with pulsed-bias pulsed S-parameter characterization is the need to apply pulses with ever-increasing amplitude (up to 200 V and 10 A) and ever-decreasing pulse width (smaller than 400 ns). The load-pull measurements can be done with a variety of setups, with active or passive approaches, and with or without handling harmonic frequencies. The challenges of load-pull system development are to offer time-domain voltage and current waveforms at the transistor terminals-an invaluable tool to provide insight in highly nonlinear transistor behavior-in addition to the capability to present low input impedances (1 Omega) and to handle high power levels (up to 100 W).