This chapter defines eco-anxiety through a detailed analysis of its major conceptual building blocks. The definition focuses on ecological risks as the object of eco-anxiety, on environmental awareness and the related feeling of insecurity, and on behavioural responses ranging from risk-assessment and risk-minimization to discouragement and paralysis. The chapter shows that eco-anxiety revolves around a core tension between certainty and uncertainty. It also explains that, even though eco-anxiety should not be categorized as a medical condition, some forms of eco-anxiety can lead to mental disorders. To help separate healthy from unhealthy forms of eco-anxiety, the chapter builds a distinction between different degrees of eco-anxiety. Finally, the chapter explains that eco-anxiety has crucial moral and political implications, as it can lead to moral injury, the violation of the human right to health, and emotional injustice.