Chapter 1 starts with describing the history of filter design. As many types of filters exist, subsequently, a classification is developed that puts all these filter types in an overall structure. It enables the designer to select the correct class of filter for his/her application. These lectures notes focus on the design of analog active time-continuous filters. This chapter describes the first step of the design methodology: transforming the differential equation describing the required filter function into a block diagram comprising ideal basic building blocks (integrators and scalers).