Background: Informal learning environments (ILEs) like Fablabs and Makerspaces have potential to facilitate development of STEM skills. However, these environments might be difficult for teachers to adopt in their teaching because of teaching approaches grounded in constructionism where the role of the teacher changes from a transmissive instructor to an active co-creator, and using high-tech equipment not normally found in schools. Purpose: The aim is to investigate teachers' self-efficacy and perceived role when teaching STEM in Fablabs and Makerspaces. This is investigated related to teaching in ILEs and using high-tech equipment. The study was conducted in two countries/regions, Flanders (Belgium) and Sweden We also compare differences between teachers depending on nationality, gender, and years of teaching experience. Sample: A total of 347 secondary school teachers completed an online survey. Quantitative analyses was used for all questions in the survey, except one open-ended question, which was analysed through inductive thematic coding. Results: The teachers reported moderate self-efficacy for teaching in ILEs , and low self-efficacy for using high-tech equipment. Some teachers described themselves as having active roles as a coach or as co-learner during visits with their students. Others saw themselves as having a passive role. Many teachers did not know what kind of role to take. The teachers who perceived an active role as a teacher in high-tech ILEs reported higher self-efficacy to teach in these environments than other teachers. Conclusions: This study shows that a constructionist approach to teaching is important if teachers are to develop self-efficacy to teach in high-tech ILEs. Thus, developing teacher practices in line with constructionism in relation to teaching in high-tech ILEs is imperative, in teacher education. The results also highlight that staff in Fablabs and Makerspaces are important for handling high-tech equipment. Hence, collaboration between staff in ILEs and teachers is of importance.