A case series is a descriptive study that follows a group of patients who have a similar diagnosis or who are undergoing the same procedure over a certain period of time. As there is no experimental protocol or control for allocation of patients to treatment, surgeons and patients decide on whether or not treatment is given, making the clinical sample representative of a common clinical population. Results of case series can generate hypotheses that are useful in designing further studies, including randomized controlled trials. However, no causal inferences should be made from case series regarding the efficacy of the investigated treatment. This article will provide principles for the design, analysis, and reporting of case series, illustrated by examples from the orthopaedic surgical literature.