A comprehensive nuclear test ban treaty that would ban all nuclear testing by all states in all environments and for all time is one of the highest arms control priorities for many states. Experts outside the US government have repeatedly questioned the need for test explosions of stockpiled nuclear weapons to assure their reliability. They maintain that the government should have a high degree of confidence in the reliability of the existing stockpile, and therefore that explosive testing is simply unnecessary. A verification system for a test ban agreement has two basic purposes: to provide confidence that other parties to the treaty are obeying their treaty obligations, and to deter parties from conducting clandestine explosions. There are two major issues involved in a negotiated nuclear test ban: technical prerequisites and political preconditions. They concern the installation of a seismic monitoring system and the role of on-site inspections to investigate suspicious seismic events.