Within the SciMaG project, the current activity is the research of threshold concepts in STEM fields. The threshold concept research in Croatia for high school mathematics teachers was conducted on June 19th, 2024. This phase included high school mathematics teachers from Rijeka and its surroundings. Around 15 teachers participated in the research, gave their professional input, and through discussion, enabled important conclusions and recommendations. In the following period, other steps in the research will follow.