This paper reports on a longitudinal study on how student teachers' understanding of the greenhouse effect developed through a teacher education programme in mathematics and science for grades 1-7 (age 7-13). All student teachers, who were accepted to the programme one year, were followed trough 2.5 years of the programme for seven terms. The student teachers took science courses in which they were taught about the greenhouse effect. They answered a questionnaire three times (n=60, n=49, n=47 respectively) in which they judged if a number of statements, about the investigated phenomena, were correct or not. In the last two questionnaires they also responded to an open-ended question about the greenhouse effect. The results show that a majority of the student teachers developed an adequate understanding of the greenhouse effect during the teaching programme. Several of the students developed further in the second science course. However a rather big group of students with poor understanding did not develop any further in the second science course and no one demonstrated full understanding. Different ways of collecting data and categorising responses affected how the students’ understanding was interpreted, which also is discussed.