Atmospheric radicals, particularly hydroxyl (•OH) and nitrate radicals (•NO3), are the drivers of chemical processes that determine atmospheric composition and thus influence local and global air quality and climate. Current techniques for measuring radicals are based on spectroscopic and mass spectrometric methods, which are technically complex, cumbersome and expensive. As a result, the measurement of atmospheric radicals is far-from-routine and only a few research groups worldwide can perform them in a very limited number of geographic locations. There is therefore a clear need to develop new radical detection techniques which are much easier to implement and deploy than existing methods. The central aim of RADICAL is to develop and deliver the science and technology to electrically detect and quantify, for the first time, short lived •OH and •NO3 radicals in the atmosphere via new, low-cost and easily accessible Si junctionless nanowire transistor (Si JNT) devices. These devices will lead to improved monitoring and control of air quality and better predictions of climate change. Three breakthrough science and technology targets have been identified: 1) fabrication and functionalisation of arrays of Si JNTs for the selective and highly sensitive electrical detection of •OH and •NO3 radicals, 2) electrical detection of atmospheric •OH and •NO3 radicals under a range of laboratory conditions and 3) evaluation and validation of the radical detectors under realistic conditions in an atmospheric simulation chamber and via deployment in the ambient atmosphere. Expertise in electronics, computer modelling, materials and surface science, nanofabrication, radical chemistry and atmospheric science is integrated on a pan-European level to achieve the overall aim of the project. This knowledge developed in RADICAL has the potential to bring about a dramatic breakthrough in air quality and climate monitoring, leading to health benefits for European citizens.