Many countries recognize the positive and effective results of improving science education through the introduction of reforms in the sciences curriculum. However, some important issues are generally neglected like, for example, the involvement of the teachers in the reform process. Taking the sciences curriculum reform under analysis and benefitting from 10 years of teachers’ experiences in teaching sciences based on this curriculum, 19 semi-structure interviews were applied so as to identify the major difficulties felt by science teachers when implementing the Portuguese sciences curriculum in the third cycle of middle school (pupils’ age range of 12–15). Some of the difficulties depicted by the data analysis include: length of the curriculum, lack of time, unsuitable laboratory facilities, insufficient means and materials for experimental work, pupils’ indiscipline and little interest in learning sciences. Although less frequently mentioned, the lack of professional development was also referred to as a constraint that seems to play an essential role in this process. Some recommendations for improving the success of sciences curriculum reforms’ implementation are given: defining and conceptualizing curricular policies by relating the reality of both the schools and the science classrooms; reorganizing and restructuring pre-service teachers’ courses; organizing professional development courses for in-service teachers.