Some argue that 21st century terrorism is no different from the past. This paper argues otherwise. It considers that our social response—The revulsion against the frequent violation of the sanctity of human life and the indiscriminate destruction of property death—Ought to be the most important factor in any attempt to categorise terrorists under international criminal law. Such a proper categorisation is as common enemies of mankind. Part 1 revisits the challenge of definition and of jurisdiction; Part 2 examines the current approaches to terrorism—Domestic, bilateral and international; and Part 3 concludes the argument whether the 21st century terrorist is a common enemy of mankind. The need for alternative social responses if the roots of modern terrorism are to be addressed is also examined.