Students’ conceptions of mathematics can influence how they participate in mathematical practices such as mathematical modelling. In this chapter we address the following research questions: What conceptions of mathematics are held by Victorian Year 11 students and how are these conceptions changed by experiencing a collaborative modelling intervention as part of their mathematics lessons? Data sources included responses to a Conceptions of Mathematics Questionnaire, Metaphor Assessment Instrument, and interviews. These were analysed for indications of Number, Components, Abstract, Modelling, and Life conceptions. Radar diagrams compared conceptions within a class and across time points. Pre-intervention there was quite a variation in conceptions in several classes with Modelling and Life conceptions usually the least developed. Post-intervention there was less variation in all classes except the least sophisticated mathematically class. There was a positive shift in conceptions after the intervention for most students but for some their Modelling or Life conceptions did not improve or narrowed.