This chapter focuses on the three weapons categories most commonly designated as Weapons of mass destruction—nuclear, biological, and chemical weapons. The massive explosive energy of nuclear weapons comes from the processes of nuclear fission and nuclear fusion. A weapon that relies on fission alone is called an ‘atomic bomb’. A weapon that relies on both fission and fusion, called a ‘thermonuclear bomb’, is much more powerful. The preferred means of delivering nuclear weapons is missiles, with ballistic missiles offering the longest range. If non-proliferation and deterrence cannot prevent the launch of a nuclear missile, a possible last-resort response is to destroy the missile before it reaches its target. Ballistic missile defence has been the subject of lengthy debates, technical and political, as to whether it constitutes a feasible and desirable response to nuclear weapons. Biological weapons are pathogenic micro-organisms deliberately disseminated to cause disease and death.