Even though the implementation of Content and Language Integrated Learning (CLIL) and its results have been researched extensively, fewer works have focused on the effect of contextual factors (CF) on teachers’ beliefs and on which ones are perceived as constraints. Furthermore, no research has explored how training might change those beliefs and help to adjust or modify some of the negative effects that CF exert on teaching practices. This qualitative study explores six in-service CLIL secondary teachers’ beliefs about CF and the effect a training course had on them. Results confirmed CF are perceived as constraints to the successful implementation of CLIL, and training appears to have a positive effect in shaping negative teachers’ beliefs and attitudes into more favourable ones. This, in turn, may help teachers to cope with the unfavourable teaching situations that CF may provoke on a daily basis. Since CF still seem to hinder CLIL success, considering teachers’ beliefs about them in CLIL teacher training programmes may contribute largely to teachers’ effectiveness.