This study aimed to explore the use of electronic monitoring within the context of anaesthetic practice. We conducted workplace observation of, and interviews with, anaesthetists and other anaesthetic staff in two UK hospitals. Transcripts were analysed inductively for recurrent themes. Whilst formal sources of knowledge in anaesthesia deal with the issue of monitoring in terms of theoretical principles and performance specifications of devices, anaesthetists in practice often ‘disbelieve’ monitoring information. They call on and integrate other sources of knowledge about the patient, especially from their clinical assessment. The ability to distinguish ‘normal’ and ‘abnormal’ findings is vital. Confidence in electronic information varies with experience, as does the degree to which electronic information may be considered ‘redundant’. We conclude that electronic monitoring brings new dimensions of understanding but also the potential for new ways of misunderstanding. The tacit knowledge underlying the safe use of monitoring deserves greater acknowledgement in training and practice.