The evasion of customs duties is a serious concern in developing countries, where tariff receipts are often important, but their collection is often problematic. We study theoretically and empirically the determinants of evasion across countries and products, based on a systematic analysis of discrepancies in trade declarations – when available – for both partners. We conclude that evasion of customs duties is greater in poorer countries, especially where the rule of law is limited. The consequences are likely to be the most serious in the poorest countries, where we find a one percentage point higher tariff to be associated on average with an understatement of imports of 1% or more. We assess some policy remedies and conclude that automated customs data treatment may be particularly useful.