This study aims at exploring the difficulties experienced by teachers currently involved in sex education and the major barriers for teachers who are not or no longer involved. It is part of the first systematic large-scale survey of the practice of school-based sex education in Flanders. A representative sample of 400 secondary schools was drawn out of a total of 1050. A response rate of 33% was obtained. The majority of the teachers were currently involved in sex education. A list of 25 items based on former research was formulated to assess perceived problems and possible barriers. Factor analyses resulted in a seven-factor structure with the group of teachers involved in sex education and an eight-factor structure for the group of teachers not involved in sex education, accounting for 59 and 71% of the total variance, respectively. Four factors appeared in the two groups, e.g. relationship variables, support, teacher and student variables. The results further suggest that the potential difficulties included in the list do not represent important problems for the teachers currently engaged in sex education. Taking this into account, structural and support related problems score relatively highly. For teachers not currently involved in school-based sex education, the barriers seem to be harder to surmount. Course related variables seem to be most prominent.