This paper estimates the volume of informal trade between Algeria and Mali and analyzes its determinants and mechanisms, using a multi-pronged methodology. First, the authors discuss how subsidy policies and the legal framework create incentives for informal trade across the Sahara. Second, the authors provide evidence of the importance of informal trade, drawing on satellite images and surveys with informal traders in Mali and Algeria. The authors estimate that the weekly turnover of informal trade fell from approximately United States (U.S.) 2 million dollars in 2011 to U.S. 0.74 million dollars in 2014, but continues to play a crucial role in the economies of northern Mali and southern Algeria. Profit margins of 20-30 percent on informal trade contribute to explaining the relative prosperity of northern Mali. The authors also show that official trade statistics are meaningless in this context, as they capture less than 3 percent of total trade. Finally, the authors provide qualitative evidence on informal trade actors and mechanisms for the most frequently traded products.