This chapter aims to describe methodologies and techniques for de-embedding device measurements from extrinsic measurements by characterizing the parasitic network surrounding the intrinsic device, through the use of a three-dimensional (3D) physical model of the network and its electromagnetic (EM) analysis. The electromagnetic behavior is obtained employing 3D EM solvers and internal ports. In the first part, the de-embedding processes for field-effect transistor (FET) devices to be used for monolithic microwave integrated circuit designs are studied by four different approaches; in the second part of this chapter, the de-embedding of FET devices for hybrid circuit design purposes is described. © 2014 Elsevier Ltd. All rights reserved.