Summary This paper gives an overview of the Emerald (Emerging mental health systems in low- and middle-income countries) programme and introduces the subsequent seven papers in this BJPsych Open thematic series. The aims of the Emerald research programme were to improve mental health outcomes in six low- and middle-income countries (LMICs), namely Ethiopia, India, Nepal, Nigeria, South Africa and Uganda, by building capacity and by generating evidence to enhance health system strengthening in these six countries. The longer-term aim is to improve mental healthcare, and so contribute to a reduction in the large treatment gap that exists for mental disorders. This series includes papers describing the following components of the Emerald programme: (a) capacity building; (b) mental health financing; (c) integrated care (d) mental health information systems; and (e) knowledge transfer. We also include a cross-cutting paper with recommendations from the Emerald programme as a whole. The inclusion of clear mental-health-related targets and indicators within the United Nations Sustainable Development Goals now intensifies the need for strong evidence about both how to provide effective treatments, and how to deliver these treatments within robust health systems.