In the 1990s, NATO began a course of enlargement and transformation to remain relevant in Europe's post-Cold-War security environment. As part of its commitment to enlargement, it admitted three new members--Poland, Hungary, and the Czech Republic--in 1999 and has plans to admit more countries in the future. NATO's enlargement has profound military implications for the United States and its allies in terms of future planning and shaping strategies. Its enlargement and its transformation, from an organization for the collective defense of its members to one whose mission includes conflict prevention and conflict management throughout Europe (including beyond its treaty area), have both been driven primarily by political imperatives--i.e., not by a sense of direct threat, but by an environment-shaping agenda of democratization and integration. This report develops and applies an analytical framework for thinking about the determinants of future NATO enlargement, the specific defense challenges they pose, and shaping policies that might aid in addressing these challenges. The approximately twelve countries that could conceivably join NATO in the next 10 to 15 years are evaluated according to political, strategic, and military (particularly airpower) criteria to determine where they stand in relation to NATO's established preconditions for membership consideration and NATO's strategic rationale for issuing invitations to join. The result is a rating of each potential member's relative readiness for and likelihood of acceding to NATO.