The world’s forests are biologically diverse terrestrial ecosystems, defined by their dominant vegetation and the organisms living in them. They are habitat for some of the Earth’s greatest biological diversity, but are being increasingly threatened through human activities including deforestation, fragmentation, and climate change. (Convention on Biological Diversity Urban forests face the most severe pressures due to intense global urbanization. Key to their survival is better knowledge about what constitutes a healthy urban forest, both in terms of reference from the past and potential design into the future.