This chapter covers the initial steps in exposure measurement that should occur during the planning of an epidemiologic study of the relationship between an exposure (risk factor) and a disease (or other outcome). This process should begin with conceptualizing the true exposure hypothesized to cause the disease. Key issues related to the true exposure include specifying the active agent and determining where in the exposure-disease biologic sequence to measure the exposure. Since the true exposure is often not measurable, the researcher needs to create an operational measure of exposure. This involves determining the appropriate exposure variable (e.g., cumulative dose of the active agent over some aetiologically important time period), the individual items that need to be collected, and the measurement instrument(s). The issues and terminology around defining the most appropriate dose representation and the most critical time window during which the exposure has the greatest effect on disease risk are discussed.