Longitudinal study designs in biomedical research are motivated by the need or desire of a researcher to assess the change over time of an outcome and what risk factors may be associated with the outcome. The outcome is measured repeatedly over time for every individual in the study, and risk factors may be measured repeatedly over time or they may be static. For example, many clinical studies involving chronic obstructive pulmonary disease (COPD) use pulmonary function as a primary outcome and measure it repeatedly over time for each individual. There are many issues, both practical and theoretical, which make the analysis of longitudinal data complicated. Fortunately, advances in statistical theory and computer technology over the past two decades have made techniques for the analysis of longitudinal data more readily available for data analysts. The aim of this paper is to provide a discussion of the important features of longitudinal data and review two popular modern statistical techniques used in biomedical research for the analysis of longitudinal data: the general linear mixed model, and generalized estimating equations. Examples are provided, using the study of pulmonary function in cystic fibrosis research. (C) 2000 Wiley-Liss. Inc.