Classification is a fundamental scientific activity and remains one of the most intractable research and applied problems in special education. This article describes the strengths and weaknesses of different models of classification (primarily monothetic and polythetic) and then focuses on cluster analysis as a method of pursuing a polythetic model. Design features that need to be incorporated in a cluster analytic study are discussed in relation to theory formulation, internal validity, and external validity. References to both methodological and applied articles are provided for illustration and further study.