This book is grounded in the author's experiences of teaching mathematics forprospective elementary school teachers and conducting research on theirunderstanding of mathematical concepts. It is a reflection on practice and anattempt to cope with a double challenge: that of a teacher, in helping prospectiveteachers make sense of mathematics, and that of a researcher, in an attempt tounderstand and describe the challenges faced by students. This work fits withinthe current community interest on teacher education and provides a novel focus,with both theoretical and practical considerations.The central claim in this book is that encounters with mathematical content by prospective elementary schoolteachers constitute relearning, rather than learning, of mathematics. The specific focus is on topics related toelementary number theory (e.g. divisibility, prime factorization), which is referred to as a "forgotten queen"(following Gauss' reference to number theory as a queen of mathematics). This is the content area that has notreceived significant attention in mathematics education research. The book can be summarized as an attempt toaddress the following questions: What is relearning of mathematical content and how is it similar to or differentfrom learning? What are the examples of specific mathematical topics or concepts that require relearning? Whatpedagogical approaches can support relearning? The detailed analysis ofresearch data and pedagogical approaches presented in the book areintertwined with stories of personal experiences of the author, which makesthe reading not only intellectually stimulating but also enjoyable.