Exposure is a substance or factor affecting human health, either adversely or beneficially. More precisely, in environmental epidemiology, exposure to an environmental substance is generally defined as any contact between a substance in an environmental medium (e.g., water, air, and soil) and the surface of the human body (e.g., skin and respiratory tract); after uptake into the body, it is referred to as dose. Exposure assessment is the study of distribution and determinants of substances or factors affecting human health. It consists of three components: the design of the study, data collection, and the interpretation of the data. This chapter discusses briefly some of the basic issues and introduces topics for the following chapters.