In this paper, which is a revised and up-dated version of an article which appeared on the back of the United States Pilot Chart of the North Pacific Ocean for May 1964, Mr. Fricker, a member of the Maritime Safety Division of the United States Naval Oceanographic Office, first of all analyses world-wide statistics of collision at sea on a geographical basis. He highlights the results of an investigation conducted by the U.S. Coast Guard into the conditions surrounding marine collisions; taking into account the results of this enquiry the author then goes into the common causes of collision. The paper then reviews the whole subject of radar and collision and the interpretation of the Collision Regulations in relation to the use of that aid. Certain collision counter-measures such as particular radar displays, improved lights and sound signals, bridge to bridge communication, &c. are then discussed.