This chapter introduces Michael Lipsky’s theory of street-level bureaucracy. It discusses the two key influences identified in the 30th anniversary edition of the book, Street-level Bureaucracy: studies of policy implementation and failure; and the institutional contexts in which street-level bureaucrats work. The theory suggests that discretion is an inevitable aspect of key public services involving interactions with clients. But it also makes clear that discretion is not simply the exercise of individual judgements or prejudices. Street-level bureaucrats make decisions in contexts that direct and limit the options available to them as they respond to the needs of individuals.