Case studies - broadly defined - have featured in human society for millennia, although the term 'case study' was first used in a scientific sense in the early twentieth century ( Gerring, 2017: xvii-viii, and see pp. 17-19 for a survey of the history of case study research). Case study research was common in the US sociological literature until the 1940s when survey research became more popular (Platt, 1992). Interest in case study research re-emerged in the 1980s as researchers recognized that statistical analyses alone could not explain the complexity and context-dependent qualities of social phenomena, and that case study research could do it better. Despite its current popularity, case study research is not well understood by social science practitioners, and it is given superficial treatment in introductory texts for university students. This chapter gives prominence to Chicago School sociology because it shaped the formative years of criminology during the first half of the twentieth century. However, we must bear in mind that as an interdisciplinary field of knowledge, criminology has been influenced by other disciplines and key texts in developing case study research. Case study research is used throughout the social sciences, and in law, business and medicine. I illustrate the differing ways it has been used in criminology and related fields. The chapter will compare different approaches to case study research, sketch its historical development in US sociology, and define key terms. A potential point of confusion, which I clarify, is how case study research relates to comparative case study research and to comparative criminology.