By complementary currency systems (CCS) we mean a specific unit of account that complements the official currency and has been developed on a group of agents that have formed a network or operate in a defined territory, with a view to accounting for and regulating exchanges of goods and services. Despite the topicality and the number of CCS, economists seem apparently pays only marginal attention to them. This article suggests that economics is based on a particular methodological and epistemological approach and on theoretical and normative conceptions of money that prevent it from taking into account the CCS's practices, their logics and their impacts. Their diversity and their relative new emergence confront economics to a methodological problem of impact studies. Because of their limited validity, the CCS tend to be considered as peripheral and transitional. Last, we show the obstacles that prevent monetary theories to recognize and legitimate them.