A frequent criticism of published surgical studies is whether the study has adequate power. This is particularly relevant for randomized controlled trials (RCTs), the preferred study design (when feasible), due to bias reduction. The credibility of the conclusions of an RCT is, to a large degree, dependent on the power of the study. The power of the study is itself influenced by the sample size. This chapter explains the necessary variables in the calculation of the sample size. Additionally, through a clinical scenario, the chapter provides the reader the necessary skills to appraise an RCT based on its sample size and power.