The book "Teacher Professionalism and the Quality of Mathematics Instruction" by Michael Besser offers a comprehensive analysis of the professional competencies of teachers and their impact on mathematics instruction. It examines how teachers' subject knowledge, pedagogical content knowledge, and beliefs influence the quality of teaching. Through quantitative studies conducted as part of the COACTIV research project, the relationship between these factors and cognitively activating instruction is explored. A particular focus is placed on teachers' knowledge about tasks, which is considered crucial for effective and meaningful teaching and learning of mathematics. This book is aimed at professionals engaged in improving the quality of instruction and teacher education.