Environmental problems, food safety issues, diagnosis of medical conditions, and the increasing threat of bioterrorism require new, fast, and cost-effective analytical techniques to monitor wider ranges of analytes in air, water, soil, and body systems and to do so with greater frequency and accuracy. Each year, numerous new compounds, with unknown effects on human health, have been developed and have eventually found their way into the environment. For example, the U.S. Environmental Protection Agency reported that in 45 of the 50 U.S. states, 141 unregulated chemicals were found in tap water, 40 of which were served to at least one million people. Good analytical devices are available now, but new devices are urgently needed to detect environmental pollutants and medical-related targets - reliably, cheaply, and rapidly. This paper explains how biosensors can meet all of these needs; describes a biosensor, its parts, and how it works; and presents case studies of possible applications .