In the early hours of 31 May 2010, Israel intercepted six vessels on the high seas carrying humanitarian aid to Gaza justifying its actions by invoking its right to enforce the blockade and prevent contraband from reaching the territory. This paper examines the interception from the perspective of international law by considering three pivotal sets of questions: (1) can Israel invoke a prima facie right to blockade Gaza? What is the legal basis for this right? What effect, if any, does the characterisation of the Israeli-Hamas conflict have? (2) If Israel does have a prima facie right to blockade Gaza, is the blockade legally constituted and maintained? What factors must be taken into consideration? Finally, (3) can Israel lawfully intercept vessels on the high seas without permission of the flag-state? In what circumstances, and under what conditions, can Israel undertake such an operation? Did Israel act lawfully when it intercepted the Flotilla vessels? The paper concludes that although it is likely that Israel’s blockade on Gaza is not prima facie unlawful, to the extent that the blockade starves the civilian population or prevents that population from receiving supplies necessary for its survival, it is undoubtedly illegal under international law.