The United States faces stealthy adversaries who have demonstrated both motives and means to inflict grave damage on the U.S. homeland. The nation's strategy in response to this type of adversary is clear: engage the threat as far as possible from the U.S. homeland, on its turf. This approach requires a multi-agency government effort, with the Department of Defense (DoD) playing a major role. A capability to protect the homeland is a necessary complement to the capability of strategic reach against these asymmetric threats. However, the challenges of homeland protection are complex. There are so many assets to protect, so many modes of attack available to adversaries, and so many organizations (federal, state, local, and private) involved that, understandably, both the conceptual thinking and the capabilities required are still immature. Responsibilities and authorities must be assigned and operative terms (homeland defense and homeland security, for example) need to be defined. The Defense Science Board (DSB) read with care current definitions and wrestled with inventing new ones. In the end, instead of focusing on precise distinctions between various terms, the board adopted a broad framework, consistent with the study terms of reference, within which to consider homeland protection issues. Maturing the conceptual framework and capabilities related to homeland protection, the DSB believes, requires a holistic approach. However, organizational boundaries inhibit such an approach. Thus, fostering a holistic approach to protecting the homeland is a guiding theme for this study and the recommendations reflect this theme.