In the context of the high-profile controversy that has unfolded in the UK around the combined measles, mumps and rubella (MMR) vaccine and its possible adverse effects, this paper addresses how parents in Brighton are thinking about MMR for their own children. Research focusing on parents’ engagement with MMR has been dominated by analysis of the proximate influences on their choices, and in particular scientific and media information, guiding policy to focus on information and education. The ethnographic work in Brighton reported in this paper, to be complemented by survey work, begins to question the validity of such reasoning by showing how wider personal and social issues shape parents’ immunisation actions. Extended parental narratives show how parents’ practices around MMR are shaped by their personal histories, by birth experiences and related feelings of control, by family health histories, by their readings of their child’s health and particular strengths and vulnerabilities, by particular engagements with health services, by processes of confidence-building and undermining, and by friendships and conversations with others, which are themselves shaped by wider social differences and transformations. “MMR talk” has become a social phenomenon. Many see vaccination as a personal decision which must respond to the particularities of a child’s immune system. These perspectives both challenge key tenets of public health policy, and suggest ways in which people’s engagements with MMR reflect wider changes in their relations with science and the state.