Documentaries both inform and entertain. ‘Defining the documentary’ explores how film can tell a story about real life honestly, in good faith. Viewers expect to be told things about the real world, things that are true, though they do not demand objectivity or comprehensiveness. Documentary conventions arise from the need to convince viewers of the authenticity of the message. Since documentaries can help us understand not only our world but our role in it, their importance is linked to a notion of the public as a social phenomenon. Three figures who launched their careers in the 1920s have shaped expectations of audiences worldwide ever since: Flaherty, Grierson, and Vertov.