Crassulacean acid metabolism (CAM) is a water-conserving strategy in which stomata open at night and carbon is stored until daytime photosynthesis provides the energy to fix it. This special issue of the Journal of Experimental Botany, edited by Hultine, Cushman, and Williams, brings together a set of papers that explore new insights into the evolution and variation of this pathway. As the editors observe, this is complex trait is ripe for comparative analysis, as it has, “>60 independent evolutionary origins, and is found in >38 families encompassing >400 genera of vascular plant species.” Furthermore, there are numerous plants that exhibit facultative CAM or variations on CAM. The issue also includes discussions about potential agricultural opportunities that can come from exploiting the water-saving properties of CAM (as examples, familiar CAM plants include pineapple, agave and vanilla). (Summary by Mary Williams) J. Exp. Bot. 70: Issue 22.