This study asks how leading countries are organising and using national and regional diagnostic testing systems for Covid19 ('testing systems') in order to reduce Covid-19 mortality per capita in their populations, to avoid or shorten 'lock-downs', and reduce economic impacts from the pandemic. We will explain how testing systems have been shaped during the pandemic, and how challenges related to testing are overcome. The research will span North America, Europe, Africa, Asia and Australia as well as the UK. The project builds on an established Covid-19 UK research and knowledge exchange hub at the University of Sussex that has been facilitating rapid dialogue and dissemination of research on Covid-19 diagnostic testing between the international research team and policy makers in the UK and beyond. The hub has a track-record of reporting results that have been widely welcomed and valued by the UK civil service and reported widely to public audiences in the media. As the pandemic continues, understanding the use of testing systems remains vital to optimise the Covid-19 response and save lives. With innovation and learning continuing (e.g. around the UK government's ambitious mass-testing programme), we propose to undertake further comparative research to share lessons across contexts. In order to support Covid-19 responses internationally, we will engage with governments and share deliverables from the early months of this 15-month project. Additionally, to inform preparations for future outbreaks and pandemics, we will contribute to national and international fora seeking to learn lessons from the current crisis.