Policy efforts directed at encouraging physical activity have had minimal success to date. Drawing on Bourdieu's theory of practice, we suggest that a social practice framing might provide useful ways of thinking about why and how some practices do and could change. This article takes three case studies of transformations in mobility practices to explore conditions of possibility for change, using a secondary analysis of qualitative data from studies on cycling in London and fell running in the English Lake District. Three modes of transformation: unthinkable, thwarted and resisted, are rooted in differential interrelationships of field, habitus and doxa in these contrasting cases. We suggest that the notion of tacit, practical knowledge is more useful to understanding why change is thinkable or unthinkable than participants’ reasoned accounts of their practice; that where new social fields are available that are congruent with habitus, change is possible and that where field and habitus are tightly aligned, the conditions of possibility for change are reduced. Efforts directed at changing practice might usefully focus not on behaviour or environments but on identifying the social fields in which mobility practices are likely to be malleable. The sociology of public health needs to focus less on health behaviour and more on social practice.