Cohort study designs have several advantages over case-control studies in terms of exposure measurement. If exposure measurement occurs before disease occurrence, cohort studies are much less prone to differential measurement error. Prospective data collection should also reduce measurement error due to poor recall of past exposures. The primary drawback of cohort studies is the large sample size leading to high data collection costs. Several approaches to reduce such costs have been discussed in this presentation, such as selection of lower cost measurement methods and fully measuring the exposure only on a subsample of the cohort (e.g., nested case-control design). However, other innovative approaches to reduce costs are needed. In addition, study reviewers should also consider that the higher costs are justified in relation to the several benefits of this study design, which include not only less measurement error, but also less susceptibility to selection bias and often the ability to study multiple disease outcomes. Improving the accuracy of exposure measurement is increasingly important for cohort studies as we move on to the study of exposures that are difficult to measure or to those with lower relative risks of disease. In such studies, attenuation of the relative risk by the effects of measurement error can lead to failure to detect an association between exposure and disease. The validity of exposure measurements could be improved by a better understanding of the biologically active agent and etiologically important time period of the exposure-disease relation, and by incorporating these into the measure. Long-term cohort studies which cover the etiologically relevant time period could improve the accuracy of measures of exposures by use of repeated biologic measures or repeated updates of self-reported exposures. Measurement error also can be reduced by judicious choice of a cohort to study and by careful attention to quality control procedures. Continued emphasis on the evaluation and improvement of the measurement properties of instruments used in epidemiologic studies will improve the validity of the results of cohort studies.