Modern-day work is a central driver of the ecological crisis and structurally inapt to support life and provide for social needs in a sustainable way and thus has to be transformed to become sustainable. This chapter argues for conceptualizing “sustainable work” as a matter of recognizing the fundamental embeddedness of all human activity within the Earth’s ecosystems and biophysical reality. This means putting the material and energy basis of work at the center of the analysis, while also taking into account the social value of work in relation to human and ecological needs. After discussing existing notions of sustainable work and related concepts, the chapter develops criteria for assessing un/sustainable work by drawing on empirical data and analyzing the main biophysical prerequisites of climate change mitigation. It then focuses on the rationale and difficulties of assessing the social value and purpose of work. Given the highly contested nature of sustainable work and the structural transformation that is necessary for work to become sustainable, the chapter also discusses the most salient political implications and institutional challenges. These reveal a considerable gap between the category’s definition and its realization, which is why for the time being work is likely to remain unsustainable.