The overarching goal of the project TRIOPS is aligned with the role of education in equipping students with the skills and knowledge needed to live and work successfully in the 21st century. Obviously, the world has changed considerably over the last decades and so have the demands students have to be prepared for via education. In the 1960ies, the majority of tasks at an average work place demanded almost exclusively routine cognitive and manual skills and was characterized by the repetitive occurrence of similar situations. Today, 50 years later, the number and importance of tasks that can be handled with routine and prototypical solutions is shrinking and our knowledge becomes obsolete faster than ever. In contrast, actively acquiring new knowledge in changing and intransparent situations and non-routine problem solving are constantly rising in importance in a number of areas, be it in occupations, in education, or in private life.A prominent representative of the skills allowing us to cope with the changing requirements of the 21st century is Complex Problem Solving (CPS). CPS highlights the skills that enable a successful interaction with problem situations that are characterized by complexity, intransparency, and dynamic features. The broad array of theoretical and empirical research on CPS has resulted in a large range of insights into the nature, assessment, and relevance of CPS and has eventually led to its inclusion in the international PISA studies assessing over 500,000 15-year-old students in over 70 countries. However, even though the importance of CPS for sustainable education is undisputed, our knowledge peters out quickly when it comes to ways of fostering this 21st century skill. That is, even though we know of the fundamental need to advance the CPS skills of students in education, our knowledge how to do so is scarce at best.To counter this lack of insights into how to train CPS skills, this project will develop a theoretically grounded training of CPS skills. To this end, TRIOPS will (1) inquire into students’ strategic behaviour, both theoretically and empirically, thereby contributing towards a deeper understanding of how students actually try to overcome the challenges of complex problem situations on a process level. Building on this foundation, we will (2) develop a palpable training of CPS skills, taking into account students’ different levels of proficiency and building on insights into the fostering of skills from instructional science and previous cognitive training efforts. Specifically, the CPS training will help students to deal with complex problems by strengthening their actual strategic behaviour and, thus, the very basis of handling complex problem situations.Building on these three directions of inquiry, TRIOPS will not only facilitate our understanding of the ways in which students interact with complex problems. The project will also lead to insights into how to support and train an important 21st century skill, thereby addressing a vital need for research in education. Ultimately, we hope that TRIOPS will contribute towards better preparing students for the challenges awaiting them in the 21st century.