In the now very broad subject of harmonics in power systems, the paper focuses on the particular option of using harmonic filtering as one of the means of keeping harmonic levels below prescribed limits. On a close examination, harmonic filter design requires very detailed consideration if filters are to maintain their harmonic attenuation characteristics throughout the very wide range of system operating conditions that might be encountered. The starting point of the procedure which is developed is that of the locus of harmonic network driving-point impedances at the point of filter connection in a network. A total filter cost function is expressed in terms of filter parameters and minimized subject to the constraints which maximum permissible distortion limits impose. The paper develops comprehensive optimal design facilities which have this foundation. Examples of filter design using the facilities are given. Their overall effectiveness is tested and quantified.