It is proposed to represent a nonlinear two-port device by a scattering parameter (S-parameter) model containing a single nonlinearity. Furthermore, it is proposed that the nonlinearity be modeled as a memoryless nonlinear function. A bipolar junction transistor (BJT) operating in the active region is suggested as one application of this modeling approach. The validity of the model is demonstrated by the comparison of measured and model-predicted data for a microwave BJT. The proposed nonlinear model is represented by a linear three-port flowgraph having one of its ports terminated in a real-valued, nonlinear reflection. The model parameters are determined from measurements of device-under-test (DUT) transmission and reflection at various input drive levels. As an illustration of its utility, the model is applied to the design of an oscillator. The measured results of a constructed oscillator are provided. A presentation of a new form of calibration for microwave measurement systems precedes the nonlinear modeling discussion. The new calibration technique combines the transmission line approach to calibration with a load-pull process common to nonlinear device measurements. A two-port, one-way measurement process obviates the need for DUT reversal. The calibrated measurement of input reflection, transmission, and load reflection is discussed. In addition, the procedure for determining the small-signal S parameters of the DUT is given. / Master of Science