John Mason has been a prominent figure in the research field of mathematics education for several decades. His principal focus has been thinking about mathematical problems, supporting those who wish to foster and sustain their own thinking and the thinking of others. Among the many markers of his esteemed career was the 1984 publication of Thinking Mathematically (with Leone Burton and Kaye Stacey). It has become a classic in the field, having been translated into many languages and in use in countries around the world. Thinking Mathematically and other writings in his substantial body of work are used with advanced high school students, with pre-service and practicing teachers, and by researchers who are interested in the nature of doing and learning mathematics. This book is t, and at the same time is, a tribute to the ermous contributions made by Mason to mathematics education. It is t a tribute book because every chapter is a report of research and thinking by the authors, t simply a statement of appreciation. All engage with how others have taken Mason's ideas forward to extend their own research and thinking. At the same time it is a tribute book. It is about how research and teaching has been inspired by Mason through his substantial opus and his vibrant presence in a network of mathematics educators.