Problem solving is a cornerstone of the school mathematics curricula in many countries, as it is an essential part of mathematical knowledge and performance. Implementation of problem solving in school mathematics is also important for meeting society’s needs with respect to work, school, and life - as well as for stimulating the interest and enthusiasm of students. Countries such as Finland, Germany, Hungary, and Slovenia have a long tradition of problem solving in school mathematics. Each country has faced its own specific challenges when adopting problem solving as a constituent part of school mathematics. But through this process they have gained experience and discovered chances for integrating problem solving into mathematics lessons. The heart of achieving this mission lies in choosing good mathematical problems. Good mathematical problems are those that are not too difficult and not too easy, are interesting, challenging, and mathematically rich. They should invite students to conjecture and to explore different strategies, support extending their existing knowledge, and allow for problem extension. In this book, the reader will find such rich mathematical problems - targeting students at different school levels - that can help cultivate a problem solving culture. The book provides the coherence and direction from different perspectives that practitioners need when integrating problem solving into their teaching practices and using problem solving to teach mathematics. We believe, that the processes which accompany problem solving, contribute to both the development of students’ mathematical reasoning and to the development of their sense of autonomy.