Preshipment inspection (PSI) programmes are implemented in many developing countries to assist governments in the collection of revenue at their borders. These programmes consist of the delegation of the inspection of imports to a private firm. To study these PSI programmes, we develop a hierarchical agency model in which the government authority can rely on two supervisors, namely the private inspection firm and the customs administration, to control importers' declarations. The government's optimal contract is fully characterized. We devote some attention to the inspection policy and its comparative statics properties. In particular, we identify the situations in which PSI programmes are revenue enhancing. We also discuss the reconciliation policy, i.e. what to do in case of conflicting inspection reports by the inspection firm and the customs administration. In the optimal mechanism, mutual supervision between the inspection firm and the customs administration is used to provide adequate incentives to all parties.