In this chapter, we are interested in the mechanisms through which some customs are “incorporated” into legal systems. In order to make our demonstration, we start by clearing up some of the conceptual confusion surrounding the use of terms related to the notion of custom, by exploring some of the jurisprudential literature that addresses the distinctions among custom, customary law, and the customary sources of law. Turning to the Islamic world and normativity, we also conduct a very broad review of studies addressing the question of ‘urf. Taking the specific case of Morocco, we then concentrate on the colonial period, colonial interest in customary law, and the rejection of such law in the nationalist discourse and in the first decades following independence. Finally, again in Morocco, we focus on the specific case of al-kadd wa’l-si‘âya, which offers a striking example of the transformation of a local custom into a positive legal provision. In conclusion, we suggest some analytical caution regarding the concept of legal hybridity in order to better ascertain the historically contingent character of positive law.