This chapter discusses the main design constraints of biosensors along some different biosensor applications. It introduces the developing technologies that promise exciting advancements in the biosensor field. One of the first and still the largest areas using biosensors is the medical field. The demand to save lives inspires the development of sensors that can measure biosignals from vital signs both in surgery and during transport to high-powered sensors that can practically create a pinpoint image of any location in the human body. A device or procedure that requires penetration into the body is referred to as "invasive." Sensors in the medical field have the advantage of collecting data from subjects that are immobile—and sometimes unconscious. The unobtrusive areas on the body to place sensors are those between joints. Mizuno et al. provides an example of sensors designed to unobtrusively collect data for human behavior measurement.