The ability to accurately measure physiological and chemical changes in the body is fundamental to the development of new diagnostic and therapeutic methods. This chapter provides an introduction to biosensors, devices which convert a biological response into an electrical signal. The interface between the biosensor and the biological system being measured is of critical importance. Awareness of the components and characteristics of a biosensors, allows correct selection of the most appropriate for clinical application. The basic components of a biosensor include a biological element and the physiochemical transducer. The transducer is a device that converts one form of energy to another. The signal produced by the transducer requires amplification and processing to correctly interpret the measurement data. This chapter also provides examples of the application of biosensors in clinical engineering, focusing on direct and indirect blood pressure measurements and also assessing lung function through spirometry. Biosensors have an enormous clinical impact both in diagnosis and therapy, as well for being critical for data collection for biomedical research.