The large family of guided missiles can be divided into two main groups: those that are aimed to hit static or slow-moving surface targets (buildings, tanks, ships, etc.) and those whose targets are airborne (and probably maneuvering) objects. Missiles of the second category are called interceptor missiles. The motion of an interceptor missile, as well as that of its target, takes place in 3-D space. The relative motion of the interceptor missile with respect to the target is also 3-D. In the equations of 3-D relative motion, bold letters indicate vectors.