Chemical weapons are weapons that exploit the toxicity of chemicals to bring about death or harm. The toxic chemicals on which chemical weapons are based are known as chemical warfare agents. This article starts by providing some definitions of chemical weapons and chemical warfare agents as well an overview of key toxicology concepts and some notes on the physicochemical properties of the agents. Subsequently, it illustrates the chemistry, biochemistry, and medical countermeasures of the main classes of chemical warfare agents, namely vesicants, choking agents, blood agents, irritants, nerve agents, and centrally incapacitating agents.