Twenty-first century skills are said being key skills for preparing students for later life. A central element of these key skills is critical thinking, which comprises using qualitative and quantitative arguments in so called decision-making situations in real life. The present chapter outlines an empirical study aiming at supporting students in building up critical thinking by fostering the ability to use qualitative and quantitative arguments when solving modelling problems (as reperesentations of real-life decision-making situations) in mathematics education. 420 students from German middle-school classes took part in the intervention study. Results point out that working on decision-making situations by using mathematical modelling can change the students’ way of using arguments and making decisions, but making informed decisions is challenging students in general.