The colonial construction of customary law has crucially shaped modern Korean civil law and jurisprudence. Following Korea's annexation by Japan in 1910, the colonial authorities imposed Japan's civil law as the general law in Korea but decreed that most private legal relations among the Koreans be governed by Korean custom. Because Choson Korea had no written body of private law, the formulation of customary law was entrusted to judges and legal scholars who ascertained and interpreted Korea's customs through Western concepts of legal rights. The fact that colonial courts persisted in the belief that customary law derived its validity from past popular behavior and that they often mingled custom with reason reveals a certain analogy with developments in Western legal history. The instrument of custom in colonial Korea fostered an atmosphere akin to judicial activism in the common law tradition and it subsequently influenced postcolonial Korean jurisprudence with its heightened conception of judicial mission to create a modern private law system. Recent decisions of the Constitutional Court of Korea indicate that colonial customary law has become a subject of not merely historical inquiry but also of critical constitutional consideration.