This article presents an international study that documented the conceptions of atomic models held by 1062 in-service high school science teachers from 58 countries. First, a previous study on pre-service science teachers’ conceptions of atomic models was successfully replicated as a pilot study with an international sample of in-service science teachers. Teachers’ conceptions were investigated by analysing their drawings of atomic models. Based on these results, a multiple-choice questionnaire was developed for the main study. This questionnaire collected data on teachers’ conceptions of atomic models, teachers’ knowledge about their students’ conceptions of atomic models, and teachers’ use of atomic models in the classroom. The results show that the teachers’ conceptions of atomic models are almost evenly distributed over six different atomic models. These models are the Bohr model, the Rutherford model, the probability model, the orbital model, the probability orbit model, and the wave model. The vast majority of teachers assume that their students’ conceptions are centred on two historical atomic models, namely the Bohr model and the Rutherford model. Furthermore, the majority of teachers prefer to use historical atomic models over modern atomic models in the classroom. However, the findings also highlight that the use of modern atomic models in the classroom is positively correlated with growing teaching experience, and that teachers’ conceptions of atomic models and their knowledge of students’ conceptions of atomic models significantly influence teachers’ classroom practice.