From the use of personal products to our consumption of food, water, and air, people are exposed to a wide array of agents each day--many with the potential to affect health. Exposure Science in the 21st Century: A Vision and A Strategy investigates the contact of humans or other organisms with those agents (that is, chemical, physical, and biologic stressors) and their fate in living systems. The concept of exposure science has been instrumental in helping us understand how stressors affect human and ecosystem health, and in efforts to prevent or reduce contact with harmful stressors. In this way exposure science has played an integral role in many areas of environmental health, and can help meet growing needs in environmental regulation, urban and ecosystem planning, and disaster management. Exposure Science in the 21st Century: A Vision and A Strategy explains that there are increasing demands for exposure science information, for example to meet needs for data on the thousands of chemicals introduced into the market each year, and to better understand the health effects of prolonged low-level exposure to stressors. Recent advances in tools and technologies--including sensor systems, analytic methods, molecular technologies, computational tools, and bioinformatics--have provided the potential for more accurate and comprehensive exposure science data than ever before. This report also provides a roadmap to take advantage of the technologic innovations and strategic collaborations to move exposure science into the future.