COVID-19 is the most significant public health emergency in more than a century, causing a global economic crisis, and with long-term repercussions across society. COVID-19 continues to claim lives, many are suffering ill health (physical and/or mental) due to the virus, and health systems struggle to recover from the massive disruption. This unprecedented crisis has highlighted the urgent need for smart investments to strengthen health system resilience – to protect people’s underlying health, through enhanced preventive care and the ability to reinforce defences in acute times, to fortify the foundations of health systems by ensuring adequate core equipment and exploiting the potential of health information, and to bolster health professionals working on the frontline by building and maintaining sufficient numbers of doctors and nurses – thereby providing countries with the agility to respond not only to evolving pandemics but also to other health and societal shocks. This report identifies a set of priority investment areas needed to strengthen health system resilience. It then produces order-of-magnitude estimates of the expected costs of such investments, drawing extensively from existing OECD data and analytical studies.