This chapter examines classical conditioning in two dipteran species, Phormia regina and Drosophila melanogaster. Learning in flies has been eagerly sought because it would provide many opportunities for fundamental biological research. The distinction between contingency and contiguity may be thought of in terms of the “information” that the presentation of a CS. The goals of behavior-genetic analysis include not only a behavioral analysis of individuals, but a genetic analysis of populations as well. Depend-ing upon the interests of a researcher, a particular genetic analysis may take different forms. Genetic analysis with conditioning as the phenotype is dependent upon the nature of the data collected; i.e., whether it is group or individual data. The form taken by a genetic analysis depends upon whether learning is inferred from group or individual data; and, with group data, whether genetic differences associated with differences in learning ability exist within the group.