Prevention measures can be classified using a diagram of four concentric circles. The outermost circle consists of intelligence work, which assists in determining the probability of a particular target being hit, likely modes of attack, and the types of countermeasures that should be adopted. The second concentric circle consists of port perimeter security, which includes high fences or hedges, patrol roads adjacent to the fence, electronic equipment on the fence, armed security guards at the gates, and gate obstacles to prevent vehicles from crashing through. The third concentric circle involves the protection of relatively small, well-defined areas such as passenger terminals, work areas for dangerous substances, and other areas vulnerable to attack. Access to such areas should be permitted only to authorized persons such as passengers, crew members, and suppliers. Protection of the ship is the fourth concentric circle of security. This should include the appointment of an officer whose primary job is to protect the ship from any terrorist sabotage attempts. For each of the concentric circles of security, this article outlines security steps that should be implemented.