Classification and nomenclature of nonexperimental epidemiological design have been a "confounded" issue. This paper discusses the problems in existing classifications of nonexperimental epidemiological design from a logical viewpoint including concept, definition and classification dimensions. The author selected, conceptualized and defined the dimensions for a study design classification scheme according to the purposes of the classification, logical consistency, and relative significance of each dimension. This classification includes ten types of basic design distinguished in terms of sampling scheme and order of hypothesized occurrence, which can be grouped into exposure-control, outcome-control and general population designs. More specific concepts may be generated from addition of an additional design features(s) to a basic design. By using the scheme, this classification distinguishes the various existing designs and clarifies some ambiguity. An example is provided to describe the classification scheme. J Epidemiol, 1994 ; 4: 113-119.