This chapter provides a definition of ecological security: a concern with the resilience of ecosystems themselves in the face of climate change. After noting antecedents to this account of security in engagement with environmental change generally and climate change specifically, the chapter goes on to outline the ethical assumptions upon which this discourse is built before defining and defending this account of the referent object of security and the nature of the threat climate change poses to it. It suggests the importance of the Anthropocene context in orienting our concern to ecosystems, noting how this focus, in turn, encourages practices oriented towards the rights and needs of the most vulnerable across time (future generations), space (impoverished and marginalized populations throughout the world) and species (other living beings).