It is an international treaty whose objective is to prevent the spread of nuclear weapons and weapons technology.Opened for signature in 1968, the Treaty entered into force in 1970. On 11th May 1995, the Treaty was extended indefinitely. A total of 191 states have joined the Treaty, though North Korea,which acceded to the NPT in 1985 but never came into compliance, announced its withdrawal in 2003. Four UN member states have never joined the NPT: India,Israel, Pakistan and South Sudan.