Summary. Starting from a broad description of analog circuit design in terms of topology design and sizing, we discuss the difficulties of sizing and describe approaches that are manual or automatic. These approaches make use of blackbox optimization techniques such as evolutionary algorithms or convex optimization techniques such as geometric programming. Geometric programming requires posynomial expressions for a circuit’s performance measurements. We show how a genetic algorithm can be exploited to evolve a posynomial expression (i.e. model) of transistor (i.e. mosfet) behavior more accurately than statistical techniques in the literature. 1