Hypertension is the most significant modifiable risk factor for cardiovascular disease and contributes to the highest global burden of disease. Blood pressure (BP) measurement is among the most important of all medical tests, and it is critical for BP monitoring devices to be accurate. Comprehensive new evidence from meta-analyses clearly shows that many BP monitoring devices (including oscillometric machines and "gold standard" mercury auscultation) do not accurately represent the BP within the arteries at the upper arm (brachial) or central aorta. Particular variability in the accuracy of BP devices compared with intra-arterial BP has been demonstrated in the cuff BP range from prehypertension to grade I hypertension (systolic BP 120-159 to diastolic BP 80-99 mmHg). This is within the BP range that is most common among people worldwide and, thus almost certainly, feeding confusion around optimal hypertension guideline thresholds. At the individual level, inaccurate BP devices have major potential consequences for best practice patient management, where underestimation of true BP is a missed opportunity to lower cardiovascular risk (with therapeutics or lifestyle) and overestimation of true BP could lead to overmedication. Each problem leads to increased cost from preventable cardiovascular events and unnecessary medications. Altogether, there is a critical need to improve the accuracy standards of BP monitoring devices. In the meantime, out-of-office BP (24 h of ambulatory BP and/or home BP monitoring) or automated, unobserved in-office BP monitoring that takes the average of multiple readings using validated devices are the best available options to determine BP control.