Martin Hollis (d.1998) was arguably the most incisive, eloquent and witty philosopher of the social sciences of his time. His work is appreciated and contested here by some of the most eminent of contemporary social theorists. Hollis’s philosophy of social action routinely distinguished between understanding (rational) and explanation (causal). He argued that the aptest account of human interaction was to be made in terms of the first. Thus he focused upon the human reasons, for, rather than upon the natural causes of, action.This volume, for the first time, brings together important essays on the work of Hollis, from many different perspectives. These include politics, sociology and economics in general; international relations, rational choice theory, constitutionalism and the rule of law as well as current concerns with relativism, Rousseauist contractarianism, ‘dirty hands’ and ‘buck-passing’.