Missile defense is a branch of military science which focuses on neutralizing threats which come in the form of missiles. Originally, missile defense focused on long-range nuclear missiles, but today a range of missiles are included in missile defense programs. Many nations have some form of a missile defense program; countries which manufacture a range of missiles tend to have more extensive missile defense programs.

There are three basic kinds of missile defense. Strategic missile defense is designed to protect a country from long-range missiles, such as inter-continental ballistic missiles. Theater missile defense is focused on short-range missiles which could be used on the battlefield or in a theater of war, while tactical missile defense deals with extremely short-range tactical missiles.

Some people prefer to classify missile defense by the trajectory stage reached by potential threats; missile defense, for example, can focus on the booster stage, the mid-course phase, or the terminal phase. Other defense programs are geared towards specific points of interception, such as inside or outside the Earth's atmosphere.

Several goals are addressed by missile defense programs around the world. The first is the ability to identify incoming missiles and to classify them, to determine how much of a threat they pose. Next is the ability to track missiles, with the theoretical capability of predicting their path. Interception and destruction are also critical to missile defense, as it doesn't do much good to know that a missile is coming if you can't do anything about it.

Since the 1940s, when several nations began producing missiles and recognizing that these weapons could be potentially devastating, the field of missile defense has expanded radically. Some missile defense programs have been heavily criticized, because they tend to eat up a great deal of military spending, often with minimal and sometimes disappointing results. Some programs have even ended up as total failures, much to the disappointment of their sponsors.

Missile defense is an interesting example of the catch-22 many militaries find themselves in. Because missiles have been developed as a military weapon, it is necessary to come up with ways to defend countries from missile attacks. However, the development of missile defense has led many nations to come up with innovative missiles which are designed to subvert existing missile defense systems, thereby feeding a need for better missile defense, which forces the development of better missiles, and so forth.