Protected areas (PAs) are the most important conservation tool, yet assessing their effectiveness is remarkably challenging. We clarify the links between the many facets of PA effectiveness, from evaluating the means, to analysing the mechanisms, to directly measuring biodiversity outcomes. The modern movement to designate protected areas (PAs) started in the mid-1800s, even if the drive to protect 'special places', such as sacred grounds or hunting areas, goes back millennia 1. If the purpose of the early PAs was to safeguard beautiful landscapes or to control access to valuable natural resources 1 , today's PAs are much more than a collection of exceptional sites. They are the main conservation tool on which hopes for stemming the relentless decline in the global biodiversity largely rest. This vision of PAs as an international cooperation endeavour goes back to the early years of the International Union for Conservation of Nature (IUCN), through the work of its World Commission on Protected Areas (WCPA), created in 1960 (ref. 1). It was consolidated in the 1992 United National Convention on Biological Diversity (CBD), which mandates each contracting party to establish a system of PAs 2. PAs remain central international conservation policy, including under the CBD 2011-2020 Strategic Plan for Biodiversity 3 , and post-2020 Global Biodiversity Framework 4. Measured by extension alone, PAs are an extraordinarily successful conservation response. Found in all countries, they currently occupy 15.0% of the global land surface and 7.4% of the world's oceans 5. Their coverage has been expanding steadily 5 , and will likely continue to do so in response to increasingly ambitious policy targets 4. Confidence on PAs as a conservation tool makes good sense. Indeed, they can counter habitat loss and degradation through restrictions to activities, such as agriculture, deforestation, urbanisation, mining and road construction, by far the major causes of biodiversity decline worldwide 6 , as well as locally attenuate pressures, such as overexploitation, pollution or invasive species. However, PAs are not all the same. Indeed, they vary substantially in the nature of intended management, from sites strictly protected for biodiversity, to multiple-use areas involving natural resource use, such as forestry or hunting 5. They also vary-perhaps even more so-in the extent to which their intended management translates into practical reinforcement, from sites with legal existence but no on-the-ground presence, to areas benefiting from substantial staff and funding 7. Other factors affect the extent to which PAs can fulfil their conservation role. For example, they may be too small, too isolated, or too few to counter the surrounding pressures, or placed in locations where their value as conservation tools is limited.