In this article we provide an overview of the different study designs commonly utilized in carrying out clinical and public health research and of the points to consider in reviewing these study designs. The design and conduct of cross-sectional health surveys, case-control, prospective, and case-crossover observational studies, and randomized controlled trials, are discussed in this review article. It is hoped that careful attention to the concerns we have raised will lead to the design and conduct of high-quality research projects and their write-up.