An anti-ship missile (AShM) is a guided missile that is designed for use against ships and large boats. Most anti-ship missiles are of the sea skimming variety, and many use a combination of inertial guidance and active radar homing. A good number of other anti-ship missiles use infrared homing to follow the heat that is emitted by a ship; it is also possible for anti-ship missiles to be guided by radio command all the way.
