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Blood pressure (BP), a key vital sign, monitors general health. Oscillometric devices are increasingly used for measurement, although their accuracy continues to be critically debated. A functional block diagram is used to review the components that affect accuracy.A block diagram is presented covering the components from cuff to algorithm. The oscillometric waveform is described, considering factors that can alter its shape. Methods used to assess accuracy, including the potential use of simulators, are described.The block diagram focuses attention on cuff, amplifier, signal processing and algorithm. The importance of correct cuff size is emphasized. Accuracy can be affected by the extraction of the oscillometric pulses and the interpolation to compensate for higher deflation rates. Modern electronic amplifiers are assumed to be stable and do not drift, an assumption largely untested. Crucial to accuracy is the algorithm, but there is no standard algorithm and limited theoretical basis, leading to significant measurement errors in groups of patients, even by approved devices. The causes are not well understood, but differences in oscillometric waveform shape between patient groups have been observed and may explain auscultatory–oscillometric differences. The ability of theoretical models to explain the effects of arterial stiffness on BP measurements is discussed. Validation remains statistical though steps have been taken to improve it.The indirect nature of BP measurement poses particularly problems for ensuring accuracy. Critical assessment has done much to improve standards, but a solid theoretical understanding of the technique has not been formulated and further work is required.