On 22nd April 1915 a greenish yellow cloud drifted across no-mans-land on the Ypres Salient - chemical warfare had begun. This book examines chemical warfare in the Twentieth Century, starting with the First World War and moving on through the inter-war period, the Italian invasion of Ethiopia and the Japanese invasion of China to the post-war period, the Korean War, the Yemeni Civil War, the Vietnam War and the Gulf War. Issues such as the non-use of chemical weapons during the Second World War and Gulf War syndrome are analysed, as are the Geneva Protocol of 1925 and the Chemical Warfare Convention of 1995. Finally, the book assesses the potential for chemical terrorism in the Twenty-first Century.