Herein a missile can be defined as a form of ammunition that propels itself in flight towards its target. Within that definition, the chief distinction to draw is between the cruise and the ballistic genre. The former is a crewless bomber dependent on air for oxygen and aerodynamic lift. The latter initially receives a boost from a rocket integral to itself. After burn-out, it is in parabolic ‘free flight’ – i.e. subject, in principle, only to gravity. However, the boost phase is not usually less than a tenth of the total flight time and, at shorter ranges, may be a much higher fraction. Atmospheric resistance has some effect, too.