Knowledge by Agreement defends the ideas that knowledge is a social status (like money, or marriage), and that knowledge is primarily the possession of groups rather than individuals. Part I develops a new theory of testimony. It breaks with the traditional view according to which testimony is not, except accidentally, a generative source of knowledge. One important consequence of the new theory is a rejection of attempts to globally justify trust in the words of others. Part II proposes a communitarian theory of empirical knowledge. Martin Kusch argues that empirical belief can acquire the status of knowledge only by being shared with others, and that all empirical beliefs presuppose social institutions. As a result all knowledge is essentially political. Part III defends some of the controversial premises and consequences of Parts I and II: the community-dependence of normativity, epistemological and semantic relativism, anti-realism, and a social conception of objectivity. Martin Kusch's bold approach to epistemology is a challenge to philosophy and will arouse interest in the wider academic world.