High blood pressure can lead to heart disease and stroke, but it often goes undetected and untreated. A new technology could change that by putting a simple blood pressure monitor in everyone’s pockets. Normally, blood pressure is measured using an inflatable cuff that presses down on an artery in the arm while a device records the effect on blood flow. Now, scientists have created a smartphone attachment that measures blood pressure directly from the finger, removing the need for any specialized equipment. The user places their finger on a pressure sensor and is guided by a chart displayed on the phone to gradually increase the applied force. Meanwhile, another sensor measures blood volume by illuminating the finger and detecting changes in how light is absorbed. Software on the phone then uses this information to determine blood pressure. The researchers tested their prototype on 30 people, and found that most quickly learned how to use it, they report today in Science Translational Medicine. Although the device was not quite as precise as an arm monitor, the scientists say its accuracy could be improved by taking multiple measurements over time. Modern smartphones already contain most of the hardware needed to transform them into portable blood pressure monitors, which would be particularly useful in developing countries where access to cuff-based devices is limited, but where smartphones are becoming commonplace.