In 2012 UK Biobank decided to add 7-day physical activity to the data collected from its participants. Through a tender and subsequent interview process, Axivity’s AX3 device was selected as the physical activity sensor. As well as being the most affordable choice for large cohort studies, the AX3 provides un-filtered actigraphy data, is fully open source, and easily deployed (and returned) by post. Using 3500 devices, UK Biobank’s data collection on the first 100,000 participants is scheduled to finish in Q3 2015. Axivity and UK Biobank have a close working relationship and are well on the way to collecting the World’s largest open physical activity dataset.