This paper describes the architecture of the Socrates toolkit for building expert systems. The authors analyse the problems associated with existing expert system tools and propose a solution based on the use of logic and meta-level inference. The abstract architecture for the toolkit is described which embodies this combination of logic and meta-level inference. This architecture can be instantiated to create a system that is specialized for a particular application. This specialization process can be seen as a methodology for building expert systems. The three stages of this methodology are discussed in detail, along with descriptions of how the Socrates toolkit supports it. The current implementation of Socrates, plus a number of applications of the toolkit are described, and the open problems are discussed.