This study reviews the results of a recent project on problem solving. Taking a developmental and structural perspective, it contrasts static, paper-and-pencil tests with interactive, technology-based tests of thinking skills, with a special reference to reasoning skills including knowledge acquisition, knowledge application, and transfer of knowledge. Hungarian students aged 11 to 17 completed problem-solving tests in static scenarios (assessing domain-specific problem-solving skills from maths and science) and in interactive scenarios (assessing domain-general complex problem-solving skills). The students were also assessed for inductive reasoning and fluid intelligence, both by first generation tests. This chapter uses the results to elicit evidence for the development of dynamic problem solving and for the relationship between those 21st-century skills. Finally, it discusses the possibility of using traditional static tests to predict performance in third generation tests measuring dynamic problem solving.