Highly qualified and competent teachers are fundamental for equitable and effective education systems. Teachers today are facing higher and more complex expectations to help students reach their full potential and become valuable members of 21st century society. The nature and variety of these demands imply that teachers, more than ever before, must be professionals who make decisions based on a robust and updated knowledge base. This publication presents research and ideas from multiple perspectives on pedagogical knowledge - the knowledge of teaching and learning - and the changing nature of the teaching profession. It provides a modern account of teachers' professional competence, and how this relates to student learning. The report looks at knowledge dynamics in the teaching profession and investigates how teachers' knowledge can be measured. It provides precious insights into 21st century demands on teacher knowledge.