New curricular orientations in the secondary schools of many Western countries invite teachers of STEM school subjects (science, technology, engineering, and mathematics) to integrate these school subjects (Czerniak, 2008). In Quebec, such interdisciplinarity is not a mere recommendation, but an official component of the curriculum (a prescription). Teachers are expected to integrate the school subjects composing the STEM subjects, and to integrate this area with other school subjects. While this interdisciplinary orientation is laudable, it is important to discover how teachers whose training is disciplinary understand this mandated interdisciplinarity and apply it in their teaching practices. Based on a survey of 245 secondary school teachers, this study shows that the interdisciplinarity practiced and described by these teachers is a superficial one and is based on links that do not enable an integration of the contributions of the subjects concerned in order to solve complex problems or achieve unified knowledge (Klein, 1985, 1990; Lenoir & Klein, 2010). These links mainly involve theme-based approaches, the contextualization of subject knowledge (relationship with life outside the school), or the mobilization, in a given school subject, of prior learning acquired in another school subject. These means of understanding and implementing interdisciplinarity can be attributed to teacher education and to the organizational context and the curricular structure of the schools. This article suggests recommendations to help overcome obstacles to understanding and implementing full interdisciplinarity as highlighted by the study in question. Moreover, it also suggests that comparative studies, along with the sharing of training experiences among teachers in different countries, might shed important light on this issue. These comparative studies would make it possible to identify the best ways to train teachers to be able to implement more-than-merely­- superficial interdisciplinary practices in their classrooms. Moreover, the analytical framework and methodology used herein, as well as the results obtained, are not limited to STEM subjects but also apply to all other school subjects. Therefore, such studies as the study we report on here should be of interest to all actors (practitioners and researchers alike) concerned with interdisciplinarity in school programs at any level anywhere in the world.