The various typos of low-pass filters which may be implemented in circuits involving only RC components and feedback amplifiers arc considered in comparison with filters designed by optimization of pass-band characteristic purely on the basis of a minimum-mcau-square-error criterion. A basic requirement observed in the optimum design is that the filter transfer functions shall be physically realizable in stable networks. The design restriction which is imposed when simplification of filter circuitry is attempted is considered in relation to a particular circuit.