Fortified boundaries are asymmetrical, physical barriers placed along borders. These boundaries are more formidable in structure than conventional boundary lines, but less robust than militarized boundaries. Their goal is to impose costs on infiltrators and in so doing deter or impede infiltration. A novel dataset of all such boundaries worldwide shows that states are constructing these barriers at an accelerating rate. More than half of barrier builders are Muslim-majority states, and so are the vast majority of targets. A multivariate analysis demonstrates that, contrary to conventional wisdom, states that construct such barriers do not tend to suffer disproportionately from terrorism, nor are they apt to be involved in a significant number of territorial disputes. Instead, differences in state wealth and migration rates are the best predictors of barrier construction. Qualitative case studies suggest that the most effective fortified boundaries are found where the initiating state controls the territory beyond a boundary that blocks the only access route into the state.