Nuclear weapons fall into two principal categories: fission (atomic or A-bombs) and fusion (thermonuclear or H-bombs). Fission weapons release energy by splitting heavy atomic nuclei, producing intense blast, heat, and prompt ionizing radiation and historically causing catastrophic urban destruction and long-term health effects. Fusion weapons derive much of their yield from fusing light nuclei (isotopes of hydrogen) under extreme temperatures and pressures; thermonuclear devices typically use a fission primary to trigger fusion and can reach yields far greater than pure fission bombs. The destructive mechanisms are overlapping—blast wave, thermal radiation, prompt and residual ionizing radiation, radioactive fallout, and, in some cases (high-altitude detonations), electromagnetic pulse (EMP). The scale and geographical character of damage differ with yield and detonation type (airburst tends to maximize blast and thermal damage while surface bursts produce more persistent fallout). Humanitarian impacts include immediate mass casualties, acute radiation sickness, long-term cancer risks, psychological trauma, displacement, and collapse of medical and social systems. Environmentally, very large exchanges raise concerns about widespread fires, atmospheric soot, and possible climatic disruption. It is important to distinguish civilian hydrogen as an energy carrier (chemical reactions or fuel cells producing water) from the nuclear isotopes used in fusion weapons—similar names, fundamentally different physics and risks. International responses include arms-control treaties, non-proliferation regimes, test bans, verification networks, and medical and civil-defense preparedness; humanitarian advocacy has reframed nuclear weapons as unacceptable due to catastrophic human and environmental harms. Risk reduction strategies emphasize securing nuclear materials, strengthening verification and diplomacy, investing in detection and public health readiness, and pursuing disarmament. The global policy challenge remains balancing deterrence doctrines with measures to reduce the likelihood of use and to mitigate consequences should prevention fail.