The main condition of sustainability of a nuclear arms control regime is the rule that every participant of the regime is sure that all other participants of the regime fulfill the obligations. A certain level of trust is absolutely necessary to make regime sustainable. It can be based on a reciprocity. For example, the principle I comply with the rules until you comply with the rules was the main source of sustainability of SALT II regime. As you remember, SALT II, the treaty on Strategic Arms Limitation didn't enter into force but the Soviet Union and the United States for some period decided to comply with the rules of this treaty, established by the treaty. But, of course, to make sure that your partner comply with the obligations of the rules established by an agreement you need to verify that somehow. Usually, in nuclear arms control treaty as well as in some other arms control treaties, the participants have a right to verify the compliance of the opposite side or other sites. But there is a problem, a nuclear sphere is a very sensitive fused. It is a sphere of a very important national secrets. How to establish a balance between transparency and state secrets and how to provide for a certain level of mutual trust? This is quite difficult. So, to do that, it is necessary to answer two questions. What to be verified and how we should verify that? There were several options or several mechanisms of verification and we cannot discuss all of them. Next week, there will be a case study which will give more detailed information on one certain type of verification system. But some general rules we can understand now. So let us try to understand what can be verified. When countries signed a treaty on arms reduction to certain level, usually it takes some time to reduce the number of weapons to this established level, and sometimes it requires a long period to reach that certain level. Liquidation of weapons, it's a expensive and long process. For example, when SALT II was signed, Russia did not fortified for a long time. And in 1997, it was necessary to make certain adaptation of the treaty and the period of liquidations of weapons was prolonged. Both countries inform each other on the existing levels of weapons and the tempo of reductions of these weapons. First of all, these information have to be verified. There were some other concrete obligations in various arms control agreements and each, this type of information, each this type of obligation was to be verified. And the methods of verification differ from case to case. If according to an agreement a country have to destroy some of its weapons that have to be verified. The opposite side have to be sure that the destroyed weapon will never be restored and returns to active service. For example, in the treaty on intermediate nuclear forces, for each system a certain specific system of certain rule of liquidations was established. For example, for the Soviets SS-12 Missiles, I quote, "Missile shall be eliminated by explosive detonation or by burning missile stages. Solid fuel, rocket nozzles and motor cases not destroyed in the process shall be burned, crushed, flattened or destroyed by explosion, and front section, minus nuclear warhead device and instrumentation compartment, minus guidance elements, shall be crushed, flattened or destroyed by explosive demolition together with a missile". That was a rule just for one type of missiles, and there were other rules for every type of Soviet and American missiles. And, it was also written in the treaty, I quote, "The parties shall have the right to conduct on-site inspections to confirm the completion of the elimination procedures. This is the first thing they wanted to verify. One of the most effective instrument of verification is a system of inspections. Inspections may be conducted by each side on the territory of the opposite side, and inspections are designed to verify the compliance of the opposite sides with the obligations on the treaty. Next week, we will use example of SALT I and SALT II treaties to understand how the system of inspections can function.