In addition to North Korea, which dropped out, three countries have not joined the Nuclear Non-Proliferation Treaty: India, Pakistan, and Israel.
India for many years challenged the nuclear non-proliferation regime by saying that they would only join if they saw bona fides that you were going to move towards disarmament. They said they were one-sixth of the world's population, and as long as nuclear weapons were a currency of power, they wouldn't renounce them until they knew that there was going to be universal progress.