The "gold standard" methodology for treatment efficacy research is the randomized controlled trial (RCT), which is used extensively in medical research and in other areas such as psychology. Results from a well designed and conducted RCT, which show a new treatment to be clinically and statistically superior to current standard practice, can lead to a change in clinical practice. This paper presents a tutorial on RCTs, presenting and discussing the following principles and methods: the trial protocol, choice of control treatment, eligibility criteria, random allocation, outcomes and hypotheses, sample size, subject recruitment, analysis by intention to treat, interim analysis, stopping rules, safety data monitoring/trial management, and study documentation. RCTs are a complicated and logistically involved methodology. Hence, prior to the commitment of resources to such a trial in the development of a new treatment, the safety and clinical promise of the new treatment needs to be demonstrated with preliminary studies. Such preliminary studies have been completed for the Lidcombe Program (LP) of early stuttering intervention, and an RCT of that treatment is being conducted at the time of writing. The principles and methods of RCTs are illustrated with reference to that study. Educational objectives: The reader will understand the design principles and methods of RCTs. The reader will understand the logistics of conducting a RCT of a treatment for early stuttering.