Here we review a simple Linear Time-Invariant (LTI) model that captures the impedance transparency properties of CMOS passive mixers, as well as filtering and noise effects and compare these analyses to measured results. We then extend this model to account for device parameters and suggest a simple metric for predicting limits to performance as a function of process and frequency. Finally we discuss the requirements such mixers place upon the LO circuits, and interactions between such circuits and performance.