Over the last two administrations, protection of the homeland has formally been the top priority of US missile defense efforts. The importance of homeland or national missile defense is in the first instance a function of strategy – the refusal to accept a relationship based on mutual vulnerability with respect to certain kinds of threats. Understanding the history of homeland missile defense development is necessary to understand both the capabilities and shortfalls of the system fielded today. To be sure, there are simply too many missile-armed actors and too much uncertainty to accept complete vulnerability. But the interceptors fielded today also remain too limited, too modest, and insufficiently reliable relative to emerging threats. Significant improvements to interceptor reliability, capability, and capacity will therefore be needed to sustain and support the current strategy.