Research has proven that teachers have a fundamental influence on student results. Moreover, effective teacher professional development is one of the key mechanisms for improving student achievement. By the most recent OECD (Organisation for Economic Cooperation and Development) definition, "Professional development is defined as activities that develop an individual's skills, knowledge, expertise and other characteristics as a teacher." The M@t.abel program suits this definition both in terms of increasing teacher subject knowledge and in terms of providing math teachers with alternative solutions and methods for presenting usual contents. Given that the effects of any professional development program on student achievement are mediated by teacher actual practices in the classroom, it is necessary to verify whether teachers enrolled in the M@t.abel program do follow the training, whether they apply the acquired knowledge in the classroom, and whether they change their attitudes and teaching methods. The study will address these issues by collecting information through specific questionnaires and the teacher's M@t.abel log-book. The difficulty in measuring the effectiveness of teaching is that "teachers have powerful effects on reading and mathematics achievement, though little of the variance in teacher quality is explained by observable variables, such as education or experience". The study rules out issues resulting from hidden heterogeneity by designing ex ante a randomized experiment, which allows the authors to compare the results of two statistically equivalent groups of participants in the most objective way possible. (Contains 2 figures and 2 tables.)