While case series are low in the hierarchy of evidence, they remain the most common study design reported in the surgical literature. Case series are often first to report landmark interventions and disease classifications for surgeons, and they remain popular in reporting rare cases and novel procedures. They have high external validity and are relatively quick and cheap to perform. Case series can be used as the foundation for a higher level of evidence study, by demonstrating feasibility of an intervention, and establishing patient and outcome parameters. Alone, they are sufficient to establish an intervention’s safety, and a test’s diagnostic accuracy. However, given the lack of a control group, they have low internal validity. The key to their appraisal is to assess them as one would the treatment group of a cohort trial. This chapter provides a guide to readers on evaluating a case series in surgery.