Experts have called for more attention to the politics, communication and ethics of screening, but there has been limited guidance on how this should be done. In this article, we propose the need for an expanded evidence base for cancer screening policy and practice. This has significant implications for cancer screening research, and for the type of research that could then be considered in evidence reviews and summaries. This approach builds on two ideas previously raised but not yet fully explored in the medical literature.1,2 The first is that we need to better understand why screening happens the way it does, sometimes apparently at odds with evidence of benefits and harms. This requires thinking about screening as a social process involving interest groups, politics, markets, consumers and health professionals. The second is that we need to think more systematically about the ethics of screening. Decades of screening guidelines and research have involved implicit moral judgements, but these have rarely applied formal ethical theory or drawn on empirical ethics research. By applying ethical frameworks and theories to guide research on moral and ethical questions in cancer screening, we will be more able to make ethical judgements on cancer screening policies and practices. We argue that more explicit and systematic investigation of the social and ethical aspects of cancer screening can support better informed and more accountable policy and practice decisions.