In this chapter, readers learn about randomized controlled trials. Topics in this chapter include appropriate contexts for RCTs, types of research questions appropriate for RCTs, hypotheses related to RCTs, and the steps of running an RCT. A detailed case study highlights the components of designing and implementing an RCT, including selecting an intervention, target outcomes, experimental conditions, identification of valid and reliable measures, inclusion criteria, recruitment and informed consent, criterion and baseline testing, random assignment, fidelity checks and monitoring conditions, monitoring participant flow, posttests, and following up, as well as a discussion of statistical, practical, and clinical significance of results.