For a highly nonlinear circuit design such as an active frequency multiplier, performing an input impedance "match" is not a straightforward problem. In this work, an analysis of nonlinear input impedance matching in active microwave frequency multipliers is presented. By utilizing harmonic balance simulation of an idealized device model, fundamental aspects of performing an input "match" are explored for classical frequency doubler and frequency tripler configurations. The analysis is then repeated using a realistic device model, verifying the efficacy of using nonlinear input impedance matching to improve the output power and return loss characteristics of a multiplier.