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Walls are extensive, ubiquitous urban ecosystems that can act as habitat for a range of different species and support non-standard cosmopolitan assemblages. Most investigations into wall ecology have focused on botanical surveys rather than testing hypotheses, but it is apparent that walls can be surprisingly diverse. They also have the potential to be ecologically engineered to encourage a greater diversity and range of species. This review considers the development of wall ecology, highlighting the key characteristics of walls that have been found to influence their ability to support species, with a focus on higher plants. It then examines the kinds of plant assemblages that are found on walls and the broader role of walls within urban biodiversity, before discussing the potential for ecological engineering of walls. Some progress has recently been made in the latter area with the installation of living walls and the …