While policy makers and scholars have long devoted considerable attention to strategies like deterrence, which threaten others with unacceptable consequences, such threat-based strategies are not always the best option. In some cases, a state may be better off seeking to give others a greater sense of security, rather than by holding their security at risk. The most prominent use of these security assurances has been in conjunction with efforts to prevent the spread of nuclear weapons. Ongoing concerns about the nuclear activities of countries like Iran and North Korea, and the possible reactions of other states in their regions, have catapulted this topic into high profile. This book represents the first study to explore the overall utility of assurance strategies, to evaluate their effectiveness as a tool for preventing nuclear proliferation, and to identify conditions under which they are more or less likely to be effective.