To drive health equity into action we need to understand the needs of health disadvantaged populations, identify barriers to equitable access to quality care, and build equity into priority setting, resource allocation and performance management. We need effective and useable equity-focused planning tools. One problem with the way health impact assessment has evolved is that it doesn’t take mental health into account well enough. In response some very interesting work has been done in England and elsewhere on mental well-being impact assessment. Not only does this build mental health into the assessment but it also emphasizes equity and the wider social determinants of health. This new primer by Nimira Lalani analyzes the potential of Mental Well-Being Impact Assessment.