Structural optimization has been used for many years as an algorithmic way to select the best structural design from a number of competing designs. Expert or knowledge based systems have been proposed as ways to include more discreet knowledge. This paper will discuss some of the recent applications of knowledge based systems to structural design. An example of an exploratory system which includes both structural optimization and knowledge based systems will be described. This system uses a knowledge based program to evaluate the results of a structural optimization program to suggest ways that the constraints might be modified if a lighter weight design is required.