As many as one-quarter to one-half of the countries in the world are, to varying extents, vulnerable to armed domestic conflict. This poses, as is widely recognized, an immediate and ongoing threat to international peace and security. Quantitative studies track the frequency of armed conflict, its intensity, patterns of termination, and consequences. The ultimate goal is to better understand—and thus better anticipate or avoid—such conflict. The authors of this report pursue the answer to a question not well addressed in the field: How do the principal quantitative measures of conflict vulnerability compare to one another?