Retrospective exposure assessment has become a crucial component in the interpretation of occupational epidemiologic results. Many advances have been made over the last 2 decades, but substantial progress is still necessary to reduce the misclassification of exposure. The efforts needed include evaluating the validity and reliability of assessment methods, better documentation of the methods, use of exposure determinants to estimate exposure levels more accurately and reliably, and an increase in the understanding of industrial hygiene and biological measurement data and questionnaires, their limitations, and how to use them best. In addition, better characterization of exposures is necessary. This need includes evaluating dermal and ingestion hazards, incorporating nonoccupational sources of exposures, particularly hobbies, evaluating the effect of multiple chemicals, and exploring different exposure metrics.