As urbanization proceeds at a globally rapid pace, remnant and regenerating fragments of habitat are becoming disproportionally important for their size. Urban woodlands are relied upon to provide environmental benefits, from migratory bird stopovers to local cooling and place-based education. Land use transformation alters soils, hydrology, connectivity, and atmosphere, extirpating many species even in these relatively undisturbed fragments of the urban matrix. Species surviving these transformations are joined in novel assemblages by an ongoing influx of introduced plants, animals, insects, microbes and pathogens from a global species pool. Efforts are now underway in temperate zone cities across the globe to improve the health and function of woodland patches within cities for the benefit of both humans and biodiversity. Understanding patterns and processes in these social-ecological systems is critical to these efforts. Urban woodlands provide an arguably unique set of ecological circumstances for study of emergent ecosystems under globally expanding urbanization and climate change.