In this work we give an overview of the requirements on nuclear command and control systems. We have found that reliable command and control systems are necessary to enforce a credible operational nuclear weapon capability, while using nuclear weapons as a political instrument does not require a functional command and control system. In this report we discuss how different strategic nuclear doctrines and postures are translated into technical specifications concerning the architecture and functions of the nuclear command and control system. In particular, we have studied how a credible second-strike capability may be maintained. The necessary level of sophistication of the command and control system, defined in a wide sense, depends on the time frame of retaliation after a hostile nuclear attack. The requirements on a nuclear command and control system are different from the demands on a conventional command and control system in a number of ways. The most important difference we have found is that the political control is tighter. In all studied cases a decision to launch a nuclear attack can only be formed at the political level. Another aspect that is particularly important for maintaining a nuclear secondstrike capability is the protection of the command and control systems. Depending on the nuclear posture, the reaction time before a second-strike can be launched may be more or less important as is the availability of an early warning system.