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This chart, prepared by the Pentagon's Ballistic Missile Defense Organization, shows how military planners are thinking about layers of defensive systems deployed in any given region. Five TMD systems are in various stages of research and development. They are the PAC-3 system, the Navy Area Wide, the Navy Theater Wide, the Air Force's Theater High Altitude Area Defense, and the Air-borne Laser.

Here are a few key points to remember about these systems:

1. The least capable of these systems may be operationally ready in one to three years. The more advanced systems are at least five to fifteen years away.
2. Even if and when fully deployed, these systems would not provide an impenetrable shield against attack. Military planners envision using them in combination with offensive forces sufficient to deter attack.
3. The layered concept of defense is designed to counter missiles with varying ranges. Generally, the upper-tier systems intercept longer-range missiles and the lower-tier systems are targeted at shorter-range systems.