This chapter describes a novel and valuable approach to the relationship between economic analysis and the law called “reverse engineering legal reasoning”. Social engineering conceives of the law as a means to social ends and of the economist as the technician studying to what extent laws are fit for purpose. Building on this idea, reverse engineering legal reasoning is a way to identify economic concepts that describe—are coherent with or fit—the content of legal reasoning. To do so, alternative economic hypotheses about the content of legal reasoning are formulated. On these grounds, the degree of coherence between economic concepts and legal reasoning can be made explicit. Reverse engineering legal reasoning extends the focus of positive economic analysis from the effects of the law to its content. It is useful for economists to suggest ways to increase the effectiveness of the legal system; to contribute to its functioning; as source of evidence to test economic assumptions; and to solve disagreements among economists, especially in relation to value choices.