The difficulties and techniques of accurately characterizing worker exposure in occupational reproductive studies are discussed. The advantages and disadvantages of different sources of exposure data are also discussed. The effects of occupational exposure to certain chemicals can include reduced libido, reduced fertility, disruption of the functional properties of sperm, fetal loss, congenital abnormalities, impaired sperm motility, menstrual disorders, childhood cancers, neonatal disorders, and fetal death. In reproductive studies, accurate identification of the specific exposure period which initiated an adverse event is often difficult because both exposure and latency periods may be very brief. It can be difficult to determine a worker's level of exposure if schedule and duties vary. The level of risk is not constant at different stages of the reproductive cycle. Both quantitative and qualitative data can be used to characterize exposure. Quantitative techniques include biological monitoring and industrial hygiene measurements. Qualitative sources include company and union work records and personal interviews. The authors conclude that selection of the appropriate study outcomes, linking exposure information with the adverse reproductive event, and determining the relevant time period of exposure may be extremely difficult, time consuming, and subject to considerable error.