The treaty on intermediate-range nuclear forces (INF) has sanctified the “zero option.” It has long been understood that it is easier to verify a complete ban on a weapon system than it is to verify a numerical limit. A complete prohibition is easier to verify because a single sighting of a banned weapon would constitute clear evidence of a violation. Moreover, a complete ban would eliminate training, testing, and repair activities that could serve as a cover for clandestine weapon deployments or could support a sudden breakout from a treaty. Although a total ban may be the best option from the standpoint of verification, this is not realistic for many weapon system. In the past, numerical limits could be verified adequately because the weapon systems in question—missile silos, bombers, and ballistic-missile submarines— were hard to conceal from national technical means (NTM) of verification (primarily reconnaissance and electronic intelligence satellites). Unfortunately, changes in technology and in the strategic environment are giving rise to new weapons whose deployment will be difficult to verify using current techniques. Mobile land-based ballistic missiles, for example, are gaining increased prominence in the strategic forces of both sides, primarily because they are less vulnerable to preemptive destruction than immobile silo-based missiles. But mobile missiles are much more difficult to count since they are designed to move around the countryside and are often hidden from view. Limits on nuclear cruise missiles would also be difficult to verify using NTM because they are small and because the conventional- and nuclear-armed versions are nearly indistinguishable. In addition, the INF Treaty is giving new impetus to the search for cooperative restrictions on the military confrontation in Central Europe, where numerical limits have been hard to agree on in part because of verification difficulties.