The idea of a system is one of the grand integrating (crosscutting) concepts that pervades all of science. In the Living Systems Module, students start by looking at Earth as the interaction of four Earth systems or subsystems—the geosphere, the atmosphere, the hydrosphere, and the biosphere. The focus of the module then turns to the biosphere as students explore the phenomenon of ecosystems and organisms in terms of their interacting parts. The driving question for the module is how can we describe Earth’s biosphere as a system of interacting parts?