This paper presents an heuristic introduction to the harmonic analysis of a class of nonstationary processes and develops the estimation techniques required for such processes in Brillinger and Hatanaka [9]. The introduction is carried out via the notion of a frequency component for both a stationary and a nonstationary process. This form of presentation has the advantage of leading directly to the definitions of power spectrum, cross-spectrum and partial cross-spectrum for stationary processes, and their moving analogues for nonstationary processes. The problem of the formulation and estimation, through harmonic analysis, of a system of simultaneous equations is also considered.