Eighty-three teachers across the curriculum were interviewed to explain their views on and approaches to, the teaching of socio-scientific controversial issues to 14-19 year olds, particularly with regard to developments in biomedicine and biotechnology. This study focused on teachers' views on the nature of evidence in controversial issues and how they deployed evidence in illuminating an issue and making judgements. Three main themes emerged: the need for facts; the reliability and validity of evidence; and the contrast between facts and values. In the first theme, three "outlier" cases suggested ambivalence about the relevance of scientific facts or knowledge for discussion of controversy. It is suggested that there needs to be more support, focus and practice in a range of contexts in the teaching of evidence in controversial socio-scientific issues and that all sources of knowledge need to be examined critically. (Contains 1 table and 2 notes.)