In 2000, the Programme for International Student Assessment (PISA) study was conducted by the Organisation for Economic Co-operation and Development (OECD). It is a worldwide study of 15-year-old students' performance in mathematics, science and reading skills. The study showed relatively poor results for German students' scientific literacy (Prenzel et al., 2001). Although the 2009 PISA study reported that these competencies had improved, there is still a demand in the German education system for the development of methods to improve student performance (Rönnebeck et al., 2010). In the light of this evidence, interdisciplinary teaching (IdT) is often considered to be a useful strategy. IdT is acknowledged as a form of education that is able to impart cross-linked and relevant knowledge to students beyond the bounds of individual disciplines (Ministerium für Schule und Weiterbildung, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, 1999). This article presents two studies examining IdT's effectiveness in biology and physical education (PE) in the sixth form. This realisation of IdT has not yet been investigated by other studies. The two studies intend to answer the question "Does IdT support the students' skills and knowledge concerning practical application and problem solving better than subject-based teaching?" The first study was conducted with six courses and 141 students, and the second study had six courses and 206 students. Both were conducted for 3 hours a week for a period of 6 weeks. In order to compare effectiveness with that of regular lessons, the studies used a so-called test and control group design. This means that a test group took part in IdT lessons as mentioned above while a control group of peers received "regular" teaching as separate subjects. The effect of different lessons on the students' skills could then be compared. The findings of the studies indicate that there was an improvement in the test groups' knowledge, transfer and problem-solving skills as well as assessment skills in comparison with the control groups. Thus, the effectiveness of IdT units in biology and PE was demonstrated in these two studies. The realisation of the IdT concept presented in this article may function as an inspiration for teachers to use cross-disciplinary teaching units in their everyday teaching.