The book is a collection of problems in school mathematics specifically written for the teachers. It is an attempt to enrich prospective and current teachers with the deep knowledge of school mathematics and to develop their reasoning and proving skills. Also, it is supposed to help them to anticipate and analyze their students’ errors and use them as teachable moments. The book is intended to be used in mathematics education courses (or professional development) for pre-service or in-service secondary school teachers. It can be used in graduate and undergraduate courses, in accordance with the orientations of different teacher preparation programs. Additionally, it can be used for the independent studies. One can also imagine situations in which teachers might use certain problems from this problem book directly in working with students, but this would constitute a supplementary use of the book.