Field Effect Transistors exhibit a variety of complicated dynamic and nonlinear interactions that this session will attempt to demystify. The dynamics include self heating, bias dependent change in trapped charge, and variations due to impact ionization. These are feedback mechanisms that contribute to intermodulation as a memory effect does. A similar contribution to distortion arising from external impedances is intimately linked to the nonlinearity of the FET. Identifying and characterizing FET dynamics and linearity is a key step in the design process, but a variety of measurement issues arise. These include extraction of intrinsic characteristics, exploration of nonlinearities across the whole spectrum, and determination of rate dependencies from small-signal and pulse data. A FET is better viewed as a nonlinear system with feedback, bias dependent rates, and high-order nonlinear conductance and charge storage with specific terminal to terminal interaction.