The second chapter of this volume focusses on crowds, both conceptually and historically. The study of crowd influence has waxed and waned over the years and has seen a resurgence of interest in topics such as identity-based social movements, street action and social media. The chapter traces this line of inquiry to the mass psychology of LeBon and Tarde, who conceived of crowds as a powerful social force that compromises Rationality and Civilisation and leads to a 'mass society' dominated by charismatic leaders. Tarde’s laws of imitation extended this inquiry of physical crowds in public spaces to that of distributed public opinion of news readers with a shared focus of attention. These notions have corollaries with contemporary theories of agenda setting, collective attention cycles, crowd sourcing and intelligence, memes and viral beliefs, stock market bubbles and social media dynamics. Much of theory of crowds oscillates between positive and negative moral assessments. The chapter concludes by considering the role of social identification in the dynamics of crowds which distributes human cognition among individual actors and determines the relationship with the leader.