Illicit financial flows (IFFs) represent a critical impediment to sustainable development, draining an estimated USD 88.6 billion annually from developing economies through trade-related channels alone. Despite growing international policy commitment to reducing IFFs, as reflected in Sustainable Development Goal (SDG) Target 16.4, the measurement of these flows remains fragmented across disconnected methodological traditions. This paper provides a conceptual review of the principal methodological approaches to estimating IFFs, encompassing capital account-based estimates, trade-based estimates, offshore wealth estimates, corporate tax avoidance estimates, forensic approaches, and emerging computational techniques. Drawing on the UNCTAD-UNODC Conceptual Framework for the Statistical Measurement of IFFs endorsed by the United Nations Statistical Commission in 2022, the paper critically evaluates the theoretical assumptions, data requirements, estimation scope, and empirical limitations of each approach. Cross-cutting challenges affecting all measurement traditions are examined, including data quality constraints in developing economies, the double counting problem, the contested boundary between illicit and illegal flows, and the disconnect between estimated flows and their predicate offences. As its principal contribution, the paper proposes a Five-Dimensional Assessment Framework for evaluating IFF measurement methodologies based on definitional coherence, data accessibility, estimation precision, cross-country comparability, and policy actionability. Six testable propositions are advanced from this framework. The paper concludes that no single methodology achieves adequate performance across all five dimensions, and that effective IFF estimation requires complementary deployment of multiple methods calibrated to the institutional and data context of each country.