Countries increasingly adopt measures to make their economy more sustainable. These measures range from pricing carbon and protecting forests to promoting renewable energy and banning forced labour. This article examines how countries can effectively enforce such sustainability requirements on imports. Based on two case studies—the EU Carbon Border Adjustment Mechanism (CBAM) and the EU Deforestation Regulation (EUDR)—and other developments in EU customs law and practice, this article shows that customs inspection of elements that cannot be physically detected in the imported product as such is neither unique nor new. Similar challenges arise when dealing with massive inflows of small e-commerce consignments or having to check the origin or value of products. Indeed, for customs enforcement more generally, the trend is one away from physical inspection of products on a transaction basis at the point of entry, toward verification and control of firms before and especially after release of the goods. To implement this ‘product-to-firm’ shift, four back-up features are, however, crucial, focusing on data, penalties/circumvention, cooperation, and resources.