This chapter assesses the connections between the environment and security by reviewing two waves of scholarship, the first mostly qualitative literature from the 1990s on the general links between environmental problems and conflict and the second generation of scholarship from the 2000s, much of it quantitative, on climate change and security. In the process, the chapter interrogates the meaning of the terms “the environment” and “security,” including efforts to broaden the meaning of security to include concepts such as human security. Where the earlier literature provided important insights on the causal mechanisms linking environmental change and conflict, it was limited in its generalizability and specification of scope conditions. The second generation of climate security research has more generalizability but has yet to tease out the causal pathways between climate change-related processes and security outcomes.