This article is the fourth in a series on the systematic review from the Joanna Briggs Institute, an international collaborative supporting evidence-based practice in nursing, medicine, and allied health fields. The purpose of the series is to describe how to conduct a systematic review-one step at a time. This article focuses on the study selection and critical appraisal steps in the process. These steps ensure that the review produces valid results capable of providing a useful basis for informing policy, clinical practice, and future research.