Previous papers in this series on evidence-based dentistry have discussed the first 2 steps in seeking answers to clinical problems formulating a clear question and strategically searching for evidence. The next step, critical appraisal of the evidence, is made easier if one understands the basic concepts of clinical research design. The strongest design, especially for questions related to therapeutic or preventive interventions, is the randomized, controlled trial. Questions relating to diagnosis, prognosis and causation are often studied with observational, rather than experimental, research designs. The strongest study design should be used whenever possible. Rules have been established to grade research evidence. This paper, the fourth in the series, presents an overview of research methodology most commonly used in the dental literature.