As anaesthetists we rely on the monitoring that we attach to our patients to be accurate, responsive and reliable. The display screen we examine closely daily with its beeps and alarms represents the processing of a large volume of biological signals of disparate types into something easily recognizable that can rapidly alert us to changes in a patient’s physiology. This article uses some common examples of physiological measurements that undergo the pathway of detection, transduction, processing, amplification and filtering before being displayed on a monitoring screen and stored. We explain key components of this pathway such as Fourier analysis, gain and analogue-to-digital conversion; and how confounding factors such as noise, interference and damping are dealt with. We also discuss some newer technologies that are finding their way into common anaesthetic practice such as depth of anaesthesia monitors and electronic anaesthesia records.