Conceptions are analyzed as being the emergences from interactions between three poles:scientific knowledge (K), values (V) and social practices (P). The teachers' beliefs and values have a direct influence on the way of understanding and teaching a topic. These beliefs must be taken into account in the content and strategies of the teacher's formation. In this paper, the authors discuss the importance of studying and analyzing these conceptions and present the results of the research on teachers' and future teachers' conceptions about sexual health education. In this topic, the conceptions could be deeply rooted not only in scientific knowledge but also in value systems and social practices, because teachers often refuse to teach the ethical, psychological, economic and social issues not strictly reducible to biology, such as sexuality health. The authors used a questionnaire prepared jointly within the European project Biohead-Citizen. (Contains 2 tables and 6 figures.)