Since the 2000s, concerns about the lack of theoretical developments in the peacekeeping literature have mostly been raised by critical scholars. On the one hand, Paris denounced the "cult of policy relevance" that led to neglect the "macrotheoretical questions about the nature and significance of these operations for our understanding of international politics" (2000: 44). On the other hand, critical theory assumes that theory is never politically neutral and that scholars should "be self-consciously theoretical and ask basic questions about what we are looking at and why, and what is excluded when we look at something in a particular way" (Bellamy, Williams and Griffin, 2010: 20). A critical approach to UN peacekeeping would then question the values and representations that inform peacekeeping and the political order that peacekeeping interventions shape, promote or sustain. Critical security studies (CSS) can be narrowly defined as gathering post-positivist analysis focused on human security and emancipation (Buzan and Hansen, 2009: 36). However, in a broader sense, CSS refers to a "reflexive field" (Salter, 2013: 1) that covers a variety of approaches ranging from critical theory to poststructuralism. It encompasses contrasting conceptions of security but that share the "assumption that security threats and insecurities are not simply objects to be studied or problems to be solved, but the product of social and political practices" (Aradau et al., 2015: 1). Applied to UN peacekeeping, CSS aims to understand how peacekeeping works in practice and its political and social implications. It questions agency and takes into account non-traditional security issues. For almost two decades, there has been a growing interest in emerging issues in peacekeeping such as health, gender, or child soldiers. Yet, the process that expands UN peacekeeping practices remains under-theorised within the peacekeeping literature, as well as within CSS. This chapter relies on CSS theoretical and methodological tools to study the specific case of the rise of environmental practices in UN peacekeeping. Since the 2000s, UN peacekeeping missions have been increasingly confronted with environmental challenges, especially as a consequence of their own expansion. This chapter explores the multiple transformations that resulted from growing environmental concerns. Not only does it discuss the impact of peacekeeping activities on the environment as an unintended consequence of the UN intervention, but it questions the discourses and power dynamics within the UN system that create the conditions for such an ecological impact. Drawing on the concepts of securitisation and environmentalisation, the chapter shows how peacekeeping has been framed as relevant to environmental policies, while contributing to a broader process of securitisation of the environment.