The purpose of the research analysed in this master thesis is to question if the model of collective planning and observing the lessons in mathematics – defined as a Lesson Study – improved their quality. The research was carried out during three lessons, preceded and followed by a collective discussion and planning. The purpose of process monitoring, comparing the notes and documents is to verify if the research question was answered. The Lesson Study is an approach to professional development of teachers that emphasizes two elements and that are: long-term praxis and implicit conviction of the efficacy of learning. The aim of the Lesson Study model is to form a school that provides high-quality learning for each and every child as well as an opportunity to develop for all the teachers all the way to become professionals. Collegial observation of the process of teaching refers to mutual observation, opening a discussion about teaching and enabling mutual reflection. Such an observation does not have a strict evaluation approach because the monitoring of the teaching process is not carried out from a stand-point of authority or power, but is based on mutual support, equality and motivation. Collegial observation of teaching crates a community of teachers who learn, meet regularly, and review their work, exchange experiences and ideas, solve common problems and prepare new approaches and methods. The aspiration of the Lesson Study model is to acquire a shared language and vision across the boundaries of departments and classrooms and to overcome existing detachments. The thesis describes the oral discussions and the activities that were carried out in the classroom, followed by an analysis in which the synopses of lesson preparation, joint planning and reflection on the lessons were concisely described.