Forests with complex three-dimensional (3D) structures are home to a disproportionately large fraction of the world’s biodiversity. They also capture and store substantial amounts of carbon from the atmosphere, making them a key ally in the fight against climate change. Despite their global importance, forests are facing severe climate-related threats (including fires, drought, and disease) and are being cleared at alarming rates due to logging and agricultural expansion. These disturbances dramatically alter the 3D structure of forests, impacting their ability to safeguard biodiversity and provide valuable ecosystem services. To understand and predict the true impact of these changes, it is crucial that we develop new approaches to accurately measure the 3D structure of forest canopies. Traditional, ground-based methods struggle with this, attempting to describe structural complexity from over-simplified features of their trees. However, with recent advances in LiDAR technology, we can measure complex forest ecosystems in 3D over vast areas. This technology is changing the way we understand structural complexity and what it means for the health of our ecosystems. As these methods continue to improve – becoming more robust and generalisable – they will help us better understand, conserve and restore our globally important forests.