Opponents of the nuclear industry often claim that material in transit is vulnerable to attack by terrorist groups who might seek to take the cargo and make a bomb, or otherwise cause a radiological incident. This paper lays out a range of conceivable objectives or attack scenarios in the context of maritime transportation and examines these in the light of what is known of the relevant security arrangements. It concludes that these arrangements provide a very substantial deterrent to any attempt at diversion or sabotage and that any such attempt would have little prospect of success, beyond the immediate media impact of whatever might have occurred.