Mixed methods research mix quantitative and qualitative research methods. The philosophical foundations of mixed methods research have been widely debated. In this chapter, we argue that Evidential Pluralism provides new philosophical foundations for mixed methods research in the social sciences. After giving an account of the context within which mixed methods research emerged, we explore the question of its philosophical foundations. We sketch standard approaches to the foundations of mixed methods research and note certain limitations. We then argue that the account of causal enquiry at the heart of Evidential Pluralism requires a thorough consideration of both quantitative and qualitative methods and so provides strong motivation for mixed methods research in causal enquiry. Evidential Pluralism can also provide practical guidance on how to integrate quantitative and qualitative studies.