This chapter aims to provide policy makers, researchers, organizations, and individuals working in the public health sphere with an understanding of the evidence on how factors at work might influence mental wellbeing. It uses the Whitehall II study, one of the leading cohort studies in the field, as an illustrative example. This chapter provides an overview of major theories, describes the assessment methods used to measure mental wellbeing in the Whitehall II study, and reviews the main research findings. Finally, it discusses next steps for research, including suggestions for evidence-based interventions to minimize adverse effects on mental health and promote wellbeing at work.