In this chapter, we discussed the input and output relationship of systems such as electrical circuits that are governed by differential equations. We established convolution as a method of solving the equations, and the discrete time convolution was discussed as a possible solution where a computer may be used for processing the input and output. We analyzed the series and parallel electrical networks for their varying responses by solving the corresponding differential equations. The goal in this chapter was to discuss the method of convolution as a solution of differential equations (it will form the basis of our filter design in the coming chapters) and to see the expected response of systems that are governed by differential equations.