This study describes how medical and nursing students develop their conceptions and understanding of science during 3 years of study at the academic level. The point of departure is the students' commonsense conceptions at the start of the undergraduate programme, which are seen as alternative ways of thinking to the more theoretical explanatory models conveyed in the teaching. The theoretical basis for the study is Piaget's theory of cognitive development, where cognitive structures are constructed in interaction with the individual's surroundings. How do these structures develop and how is this development related to the educational setting? A qualitative method of analysis is used, that is the focus is on the students' verbal actions and the meaning of these actions. The analysis is based on written essays, interviews, and observations. The participants are eight medical and seven nursing students studied over a 3 year period. The results indicate that whatever conceptions of science the students held at the beginning of their undergraduate programmes, they generally held the same conceptions in subsequent rounds of data collection, however these conceptions had become increasingly developed and elaborated during the course of their studies. The analysis also indicates that the educational discourse was the main reason for the students' conceptual development.