This study investigated prospective secondary science teachers' understandings of and intentions to teach about scientific modelling in the context of a model-based instructional module. Qualitative methods were used to explore the influence of instruction using dynamic computer modelling. Participants included 14 secondary science prospective teachers in the USA. Research questions included: (1) What do prospective teachers understand about models and modelling in science? (2) How do their understandings change, following building and testing dynamic computer models? and (3) What are prospective teachers' intentions to teach about scientific models? Scaffolds in the software, "Model-IT", enabled participants to easily build dynamic models. Findings related to the process, content, and epistemological aspects of modelling, including: (a) prospective teachers became more articulate with the language of modelling; and (b) the module enabled prospective teachers to think critically about aspects of modelling. Still, teachers did not appear to achieve full understanding of scientific modelling.