This chapter introduces some basic physical principles that contribute to the function of various monitors used in anaesthetic practice. Topics include biological signal processing, operational amplifiers, including single-ended amplifiers, and the benefit of patient-isolated differential amplifiers; it includes filtering, digital processing, and electrodes. The generic principles of transducers are introduced, including resistive, capacitive, and inductance strain gauges used in transducers, photoelectric, piezoelectric, and chemical transducers, calibration of transducers, and the significance of resonance and damping in measurement systems. Since both are widely used in monitoring systems, there is an introduction to spectroscopy and magnetism.