An understanding of how staff identify, classify, narrativise and orient to patient safety risks is important in understanding responses to efforts to effect change. We report an ethnographic study of four medical wards in the UK, in hospitals that were participating in the Health Foundation's Safer Patients Initiative, an organisation-wide patient safety programme. Data analysis of observations and 49 interviews with staff was based on the constant comparative method. We found that staff engaged routinely in practices of determining what gets to count as a risk, how such risks should properly be managed, and how to account for what they do. Staff practices and reasoning in relation to risk emerged through their practical engagement in the everyday work of the wards, but were also shaped by social imperatives. Risks, in the environment we studied, were not simply risks to patient safety; when things went wrong, professional identity was at risk too. Staff oriented to risks in the context of busy and complex ward environments, which influenced how they accounted for risk. Reasoning about risk was influenced by judgements about which values should be promoted when caring for patients, by social norms, by risk-spreading logics, and by perceptions of the extent to which particular behaviours and actions were coupled to outcomes and were blameworthy. These ways of identifying, evaluating and addressing risks are likely to be highly influential in staff responses to efforts to effect change, and highlight the challenges in designing and implementing patient safety interventions.