Attention to the concept of illicit financial flows (IFFs) is growing in the political debate together with the connected fight against harmful activities as tax avoidance, money laundering, drug trafficking, corruption, and terrorism financing. While the concept was introduced in the context of development economics, it is now used also in the field of tax studies, criminology, and policy evaluations. Still, there is ambiguity with respect to the actual interpretation and understanding of IFFs. This chapter provides an overview of how the literature defines and conceptualizes IFFs and, ultimately, advocates for their partial rethinking and clarification.