In this chapter, first the parametric principle is illustrated by two simple examples, one mechanical and one electrical. Then the realization of time varying reactances is explained, followed by a short history of "parametric electronics." This survey demonstrates the importance of parametric circuits in the field of low-noise microwave electronics as well as explains the organization of this book. 1.1 The Parametric Principle An oscillating system comprising a single or several time-varying energy storing elements is called a parametric system; usually the variations are harmonic functions...