Knowledge-based systems that have made the computer-aided design (CAD) of analog circuits feasible are discussed. The three systems-Idac, Oasys, and Opasyn-were formally announced in 1987. Although they differ widely in philosophy, all use common building blocks and produce sized-schematic diagrams showing how transistors, capacitors, and so forth are connected, complete with the components' values-from which custom circuits can be synthesized. These building blocks, however, unlike the ones used in semicustom analog IC design, are not fixed designs from a library. Rather, they can be varied infinitely, according to rules given to the tools by human experts, so that they approach the ideal performance far more closely than is possible with a limited choice of fixed blocks. All told, Idac, Oasys, and Opasyn can automatically synthesize analog circuits from 13 classes of analog building blocks and can produce over 100 distinct circuit topologies. The characteristics of the three systems are discussed and compared.