Case-control studies are one of the two main types of observational study designs in epidemiology. This design is desirable when exposure data are difficult to obtain, the disease is rare or has a long induction period, or when the underlying population is dynamic. In a case-control study, first patients are selected based on their disease (outcome) status and classified as cases or controls. Then investigators obtain and compare the exposure histories between groups. A case-crossover study is a specific type of case-control study where patients serve as their own controls. This is appropriate for transient exposures, where a patient contributes time as both a case and a control.