In this chapter, we will discuss the implementation of the analog approximation functions that we developed and verified in previous chapters. There are several methods that could be used to allow the successful implementation of an analog filter. Unfortunately, an entire book could be devoted to this topic (and several have), so we will concentrate on active filters. Using active filters to implement the transfer functions is a very popular method today because of the natural correspondence between the analog circuit and the mathematical function. We will not discuss the derivation of the transfer functions for active filters because the development of such circuit analysis techniques is beyond the scope of this text. However, a number of suitable references are given in the analog active filter texts of Appendix A for those who are interested in the derivations. Instead, the circuit topology and transfer function for several common active filters will be presented before determining the component values for each circuit. We will develop implementation procedures for each of the filter selectivities discussed in previous chapters as well as discuss other options for implementation and some of the important implementation issues.