This article develops a social representational approach to understanding expert knowledge and its relation to health. Research with homelessness professionals (HPs) working in the UK voluntary sector shows that expert definitions of homelessness can either undermine or enhance the health of the homeless. Guided by the concepts of social representations and cognitive polyphasia, the analysis reveals a contradictory field of knowledge, which reflects the struggle of professionals to sustain a humanizing approach and resist the pressures of statutory agendas. We conclude pointing to the need of recognizing the impact of professional's knowledge on the health and care policies for the homeless.