Any sustainability assessment uses some notion of sustainability as a reference. It is increasingly acknowledged that sustainability is a contested concept and that its different definitions are rooted in different values. Accordingly, researchers have started to develop approaches for producing knowledge in a context of diverging values. However, little attention has been paid to these values and to how they play into sustainability assessments. In this chapter, we address this question in three parts. First, we show how different ethical positions and worldviews enter notions of sustainability and shape the solution spaces for sustainability issues which are taken into consideration. Secondly, we present different stances on how the scientific method should address the presence of values and show how they can be traced back to different values about what good science is. Thirdly, we show that values are historically constructed and shaped by the socioeconomic, socio-technological, and socio-ecological order and at the same time contribute to reproducing it. We also show that this process risks leading to a prevalence of notions of sustainability that reproduce the very dynamics at the origin of the sustainability problem at hand. We conclude the chapter with recommendations for how this challenge could be overcome.