The role of optimisation in the field of circuit design is fully discussed, using some actual designs to illustrate the principles involved. A review of the more important optimisation methods is presented, and comparative tests have been carried out on realistic circuit designs. The design of these circuits would have proved difficult, if not impossible, by conventional methods. Both least-square and minimax-error criteria are discussed, and methods for dealing with constraints are considered. Attention is drawn to the partial-noise-immunity property of the simplex algorithm and its use as the basis of an experimental procedure.