This entry in the Legal Theory Lexicon provides a short introduction to the idea of functionalist explanation in legal theory. Functionalist explanations are familiar from biology, where evolutionary theory explains the existence of a trait in an organism by the effect the trait has on the ability of organism to reproduce. Sociologists frequently explain (or, perhaps, "formerly explained") social behavior on the basis of the social function that the behavior serves. Why does this group do a "rain dance"? Because the rain dance ritual serves to create social cohesion in times of stress.In legal theory, functionalist explanations explain the existence of legal rules or institutions in terms of the effects they produce. For example, Marxist explanations might explain the existence of a rule by showing that the rule serves the interests of the capitalist class. This Lexicon entry investigates the validity of such explanations and the related notions of microfoundations and methodological individual.