This book examines the state of the nuclear non-proliferation regime and the issues it faces in the early 21st century. Despite the fact that most countries in the world have signed the Non-Proliferation Treaty (NPT) there is growing concern that the NPT is in serious trouble and may not be able to stop the further spread of nuclear weapons. If so, international stability will be undermined, with potentially disastrous consequences, and the vision of a nuclear weapon-free world will become utterly unrealistic. More specifically, the NPT is exposed to four main challenges, explored in this book: challenges from outside, as three countries that have not signed the Treaty -- Israel, India and Pakistan -- are known to possess nuclear weapons; challenges from within, as some countries that have signed on to the Treaty as non-nuclear weapons states have nevertheless developed or are suspected to be trying to develop nuclear weapons (North Korea and Iran being cases in point); challenges from below in the shape of terrorists and other non-state actors who may want to acquire radioactive materials or even nuclear weapons; and, finally, challenges from above due to the perceived failure of the five legal nuclear weapons states to keep their part of the 'double bargain' made by the parties of the NPT and take serious steps towards nuclear disarmament. This book will be of much interest to students of nuclear proliferation, international security, war and conflict studies and IR in general.