Collision statistics indicate that many collisions at sea occur after one or both vessels take action to avoid collision. It is worth considering why this should be so. Is the action taken contrary to the requirements of the Regulations for Preventing Collisions at Sea or are the Rules unsatisfactory in not stating clear and effective requirements? It is necessary to consider the principles of collision avoidance to determine whether a more effective solution can be obtained. This investigation relates to all types of craft, not necessarily only marine craft.