Objective To explore what Australian experts value in breast screening, how these values are conceptualised and prioritised, and how they inform experts' reasoning and judgement about the Australian breast-screening programme. Design Qualitative study based on interviews with experts. Participants 33 experts, including clinicians, programme managers, policymakers, advocates and researchers selected for their recognisable influence in the Australian breast-screening setting. Setting Australian breast-screening policy, practice and research settings. Results Experts expressed 2 types of values: ethical values (about what was good, important or right) and epistemological values (about how evidence should be created and used). Ethical values included delivering benefit, avoiding harm, promoting autonomy, fairness, cost effectiveness, accountability, professionalism and transparency. Epistemological values informed experts' arguments about prioritising and evaluating evidence methodology, source population and professional interests. Some values were conceptualised differently by experts: for example, delivering benefit could mean reducing breast cancer mortality, reducing all-cause mortality, reducing mortality in younger women, reducing need for aggressive treatment, and/or reassuring women they were cancer free. When values came into conflict, experts prioritised them differently: for example, when experts perceived a conflict between delivering benefits and promoting autonomy, there were differences in which value was prioritised. We explain the complexity of the relationship between held values and experts' overall views on breast cancer screening. Conclusions Experts' positions in breast screening are influenced by evidence and a wide range of ethical and epistemological values. We conclude that discussions about values should be a regular part of breast-screening review in order to build understanding between those who hold different positions, and provide a mechanism for responding to these differences.