The world is facing a biodiversity crisis and around 1 million animal and plant species are estimated to be threatened with extinction (IPBES, 2019). Trees are highly visible in most landscapes and are excellent biodiversity indicators. They are ecologically, culturally and economically of vital importance and yet there have been surprising gaps in knowledge of the diversity, distribution, abundance and conservation status of trees at a global scale. This report presents a summary of what we do know about tree species around the world. It presents the results of the Global Tree Assessment (GTA) which has involved five years of intensive collaborative research. The Global Tree Assessment has developed a strategic approach to data collection on tree species involving an extensive global network of organisations and individual experts. The network coordinated by BGCI, working in partnership with the IUCN Species Survival Commission Global Tree Specialist Group, aims to complete comprehensive conservation assessments on the IUCN Red List of all tree species by 2023, building on and amplifying earlier initiatives.