Although the benefits of the use of simulations in science education have been extensively documented, research on pre-service teacher education related to the use of simulations in science teaching remains limited. The aim of this study was to investigate the beliefs of pre-service primary teachers in two teacher training programs of two different universities (n = 36 and n = 18) related to teaching science with simulations. The teachers participated in an intervention where they planned and gave a science lesson where simulations were used. The effect of the two different types of interventions on the beliefs was also studied. The Interconnected Model of Professional Growth by Clarke and Hollingsworth is used as a framework for the effect that the intervention has on the beliefs. The data was collected through post-intervention surveys with open questions. After the both interventions pre-service teachers perceived the simulations’ ability to demonstrate otherwise unobservable phenomena and motivate the learners’ as their advantages and appropriate use of simulations in relation to the learning goals was seen a challenge. Likewise, all pre-service teachers viewed technological and pedagogical knowledge as important know-how for teachers when teaching with simulations. There were differences in the conceptions after the two interventions, mostly related to the weaknesses of simulations and the teacher know-how needed. These can be explained with the differences between the interventions. The results confirm the impact that external stimuli such as these kinds of interventions have on teachers’ beliefs. It is vital to design teacher training for simulations in a way that offers just the right amount of support to enable the future teachers to be able to start teaching science with simulations.