From its beginnings in Germany in the twentieth century, a thriving extensive green roof industry has become established in many countries in temperate climates. Based upon the success of the industry, and with an expectation that this technology will be adopted in other climates, this review of the ecological research of extensive green roofs aims to evaluate the application of this knowledge. The modern extensive green roof is the product of research in the 1970s by German green roof pioneers; the selection of suitable species from analogue habitats led to green roof vegetation dominated by drought tolerant taxa. The commercial success of extensive green roof systems can be attributed to engineering and horticultural research, to policy mechanisms in some places, and to a market that encourages innovation, and the origins in ecological design are now easily overlooked. Some of the work reviewed here, including the classification of spontaneous roof vegetation into plant communities, is not widely known due to its confinement to the German literature. By re-visiting the history of the extensive green roof and reviewing the ecological research that has contributed to our understanding of it, the intention is, for this paper, to inform those considering green roofs in other climatic regions, to apply an ecologically informed approach in using local knowledge for developing installations that are suited to the bioregion in which they occur. Finally the paper considers some future directions for research and practice.