A ballistic missile goes through several distinct phases of flight that are common to almost all such designs. They are, in order:
- boost phase when the main boost rocket or upper stages are firing;
- post-boost phase when any last-minute changes to the trajectory are made by the upper stage or warhead bus and the warheads, and any decoys are released;
- midcourse which represents most of the flight when the objects coast; and
- terminal phase as the warhead approaches its target and, for longer-ranged missiles, begins to reenter the atmosphere.
These phases are particularly important when discussing ballistic missile defense concepts. Each phase has a different level of difficulty in performing an interception, as well as a different outcome in terms of its effect on the attack as a whole. For instance, defenses that take place during the terminal phase are often the simplest to build in technical terms as they require only short-range missiles and radars. However, terminal defenses also face the most difficult targets, the multiple warheads and decoys released during the post-boost phase. In contrast, boost-phase defenses are difficult to build because they have to be located close to the target, often in space, but every success destroys all of the warheads and decoys.