This book on thermodynamics in ecosystems deliberately promotes a novel approach, by emphasizing the physical concept of action, to complement that of energy. It aims to show that too much attention may have been devoted to energy in biothermodynamics and insufficient attention to action. This relative neglect may now be limiting our capacity to understand how ecosystems function, how they evolved and if they can be sustained, as human demands for food and shelter increase. The idea that an appreciation of action will be needed in future to ensure a proper integration of physical, chemical and biological phenomena essential to sustain and safeguard the Earth's ecosystems is developed. The significance of action is made clear by examination of case studies related to key processes in living systems.