This chapter covers the current gold standard for evaluating the effectiveness of therapeutic interventions, the randomized controlled trial (RCT). Key features of the RCT, regardless of sub-type, are randomization, allocation concealment, and blinding. These key features help reduce bias and the influence of confounding variables, making the randomized controlled trial eminently suitable to determine cause and effect relationships. Protocol design and registration prior to trial onset are important factors in determining the quality of the trial, and various trial design sub-types, including parallel, factorial, crossover, and cluster, are outlined and the strengths and weakness of each examined. Various checklists such as SPIRIT and CONSORT can be used to ensure proper reporting of both trial protocols and trial findings, to ensure clear, concise reporting. Finally, the shortcomings of RCTs and newer trial designs, such as comparative effectiveness research and pragmatic studies, designed to overcome some of these issues are examined, and ways to make clinical trial results more clinically applicable are discussed.