This chapter explores what a critical approach to UN peacekeeping entails and highlights the valuable contributions of Critical Security Studies (CSS) to capture the nature and significance of peace operations in international politics. It shows how CSS questions the values and representations that inform UN peacekeeping and the political order that peacekeeping interventions shape, promote or sustain. It further discusses how CSS unpacks peacekeeping (often mundane and daily) practices and their political and social implications and takes into account non-traditional security issues. The chapter then relies on CSS theoretical and methodological tools to study the specific case of the rise of environmental practices in UN peacekeeping. Drawing on the concepts of securitisation and environmentalisation, it demonstrates how UN peacekeeping has been framed as relevant to environmental policies, while contributing to a broader process of securitisation of the environment.