This paper takes as its starting point the assumption that the 'Epidemiological Imagination' has a central role to play in the future development of policies and practice to improve population health and reduce health inequalities within and between states but suggests that by neglecting the contribution that qualitative research can make epidemiology is failing to deliver this potential. The paper briefly considers what qualitative research is, touching on epistemological questions--what type of "knowledge" is generated--and questions of methods--what approaches to data collection, analysis and interpretation are involved). Following this the paper presents two different models of the relationship between qualitative and quantitative research. The enhancement model (which assumes that qualitative research findings add something extra to the findings of quantitative research) suggests three related "roles" for qualitative research: generating hypothesis to be tested by quantitative research, helping to construct more sophisticated measures of social phenomena and explaining unexpected research from quantitative research. In contrast, the Epistemological Model suggests that qualitative research is equal but different from quantitative research making a unique contribution through: researching parts other research approaches can't reach, increasing understanding by adding conceptual and theoretical depth to knowledge, shifting the balance of power between researchers and researched and challenging traditional epidemiological ways of "knowing" the social world. The paper illustrates these different types of contributions with examples of qualitative research and finally discusses ways in which the "trustworthiness" of qualitative research can be assessed.