The concept of the case is a basic feature of social science research and yet many questions about how a case should be defined, how cases should be selected and what the criteria are for a good case or set of cases are far from settled. Are cases pre-existing phenomena that need only be identified by the researcher before analysis can begin? Or are cases constructed during the course of research, only after analysis has revealed which features should be considered defining characteristics? Will cases be selected randomly from the total pool of available cases? Or will cases be chosen because of their unique qualities? These questions and many others are addressed by the contributors to this volume as they probe the nature of the case and the ways in which different understandings of what a case is affect the conduct and the results of research. The contributors find a good deal of common ground, and yet they also express strikingly different views on many key points. As Ragin argues and the contributions demonstrate, the work of any given researcher is often characterized by some hybrid of these basic approaches, and it is important to understand that most research involves multiple definitions and uses of cases, as both specific empirical phenomena and as general theoretical categories.