Increasing reliance is being placed on the use of quantitative epidemiological methods in the conduct and evaluation of pediatric research. The basic design features of two common types of observational studies, the case-control study and the cohort study, are reviewed. Advantages and disadvantages of these two study designs are discussed with emphasis on aspects such as the selection of comparison groups, avoiding selection and recall bias, gathering exposure information, controlling for potentially confounding factors, and methods of analysis. Appreciation of the salient features of these study design approaches should aid the clinician/researcher in the conduct of research endeavors as well as in critically reviewing the medical literature.