There is a worldwide effort towards the development of bioanalytical devices that can be used for detection, quantification and monitoring of specific chemical species. In this context, biosensors represent an emerging trend in the diagnostics industry. A biosensor is a device that has a biological sensing element either intimately connected to or integrated within a transducer. The aim is to produce a digital electronic signal that is proportional to the concentration of a specific chemical or a set of chemicals. Biosensors are specific, rapid, cost-effective and easy to use devices that can be employed with minimal sample treatment. Biosensors have applications in many areas such as biotechnology, healthcare, pollution monitoring, food and agriculture product monitoring, the pharmaceuticals industry and defense. This reference text is devoted to the principles and applications of biosensors and meets the needs of academic institutes, research laboratories and the rapidly developing biosensor industry.