Border agency co-operation is critical for increasing operational efficiency, facilitating legitimate trade and reducing traders’ costs at both the domestic and cross-border levels. Yet significant implementation challenges remain. This chapter explores progress to date on border agency co-operation, both domestically in terms of co-ordination among all agencies involved in the trade process at the border and across the border between agencies in neighbouring countries, whether in the form of logistics or procedural harmonisation, or integrated controls under a One-Stop Border Post. The chapter draws lessons from country experience and case studies to help identify for policy makers key challenges and factors for success.