Calcium manganese oxides have captured strong scientific interest in recent years due to exceptional catalytic activities, inexpensive synthesis methods, and constitution of earth abundant and environmentally benign elements. In particular, the catalytic properties of Ca-Mn-O systems have been recognised as a potentially useful new tool in addressing energy and environmental problems. This article presents an overview of new developments in the research on Ca-Mn-O catalyst materials. The article begins with the discussion about the CaMn4O5(H2O)(4) cluster which is situated within Photosystem II proteins in plant cells and is responsible for the natural water-splitting reaction. It then focuses on the crystal structures and synthesis routes of various Ca-Mn-O systems, followed by a critical discussion on the applications in catalytic water oxidation. Other known applications of Ca-Mn-O systems in the energy and environmental sectors are also discussed. (C) 2018 Elsevier Ltd. All rights reserved.