The international community renewed its commitments to tackle illicit financial flows (IFFs) at the fourth International Conference on Financing for Development (FfD4) held in Seville, Spain in June 2025. The complex transnational nature of IFFs make them exceedingly difficult to tackle, with governments facing a range of challenges. This paper – developed as part of UNDP’s Finance, Integrity and Governance (FIG) Initiative – focuses on key practical and technical constraints to tackling IFFs. It is intended as an aide to stakeholders engaged in the implementation of illicit finance-focused FfD4 commitments. The report identifies five constraints and offers recommendations for how these may be overcome.