Solving non-routine problems is a key competence in a world full of changes, uncertainty and surprise where we strive to achieve so many ambitious goals. But the world is also full of solutions because of the extraordinary competences of humans who search for and find them. We must explore the world around us in a thoughtful way, acquire knowledge about unknown situations efficiently, and apply new and existing knowledge creatively. "The Nature of Problem Solving" presents the background and the main ideas behind the development of the PISA 2012 assessment of problem solving, as well as results from research collaborations that originated within the group of experts who guided the development of this assessment. It illustrates the past, present and future of problem-solving research and how this research is helping educators prepare students to navigate an increasingly uncertain, volatile and ambiguous world. Chapters in this book include: (1) The development and assessment of problem solving in 21st-century schools (Beno Csapó and Joachim Funke); (2) Analytical problem solving: Potentials and manifestations (Jens Fleischer, Florian Buchwald, Detleve Leutner, Joachim Wirth, and Stefan Rumann); (3) Problem solving: Understanding complexity as uncertainty (Magda Osman); (4) Problem solving from a mathematical standpoint (John A. Dossey); (5) The PISA 2012 assessment of problem solving (Dara Ramalingam, Ray Philpot, and Barry McCrae); (6) Interactive problem solving: Exploring the potential of minimal complex systems (Samuel Greiff and Joachim Funke); (7) The history of complex problem solving (Andreas Fischer, Samuel Greiff, and Joachim Funke); (8) Empirical study of computer-based assessment of domain-general complex problem-solving skills (Gyöngyvér Molnár, Samuel Greiff, Sascha Wüstenberg, and Andreas Fischer); (9) Factors that influence the difficulty of problem-solving items (Ray Philpot, Dara Ramalingam, John A. Dossey, and Barry McCrae); (10) Assessing complex problem solving in the classroom: Meeting challenges and opportunities (Philipp Sonnleitner, Ulrich Keller, Romain Martin, Thibaud Latour, and Martin Brunner); (11) Log-file data as indicators for problem-solving processes (Nathan Zoanetti and Patrick Griffin); (12) Educational process mining: New possibilities for understanding students' problem-solving skills ( Krisztina Tóth, Heiko Rölke, Frank Goldhammer, and Ingo Barkow); (13) EcoSphere: A new paradigm for problem solving in complex systems (David A. Tobinski and Annemarie Fritz); (14) Assessment of collaborative problem-solving processes (Esther Care and Patrick Griffin); (15) Assessing conversation quality, reasoning, and problem solving with computer agents (Arthur C. Graesser, Carol M. Forsyth, and Peter Foltz); and (16) Epilogue (Beno Csapó and Joachim Funke).