Although the differential equation is a basic system description, obtaining this equation can be tedious and time-consuming. Consequently for a time-invariant system this approach is avoided in practice, except in special cases. The Fourier and Laplace transforms offer an alternative approach for characterizing and analyzing these systems. Insight into system behavior is often obtained by the transform method. These transforms change a function of one variable into a function of another variable, and, when applied to problems in the physical sciences, the transform pairs and variables may correspond to physical quantities. We assign time and frequency as the transform variables because these are the variables associated with the filtering devices considered in this text.