In this article we explore young children's development of mathematical knowledge and reasoning processes as they worked two modelling problems (the Butter Beans Problem and the Airplane Problem). The problems involve authentic situations that need to be interpreted and described in mathematical ways. Both problems include tables of data, together with background information containing specific criteria to be considered in the solution process. Four classes of 3rd-graders (8 years of age) and their teachers participated in the 6-month program, which included preparatory modelling activities along with professional development for the teachers. In discussing our findings we address: (a) Ways in which the children applied their informal, personal knowledge to the problems; (b) How the children interpreted the tables of data, including difficulties they experienced; (c) How the children operated on the data, including aggregating and comparing data, and looking for trends and patterns; (d) How the children developed important mathematical ideas; and (e) Ways in which the children represented their mathematical understandings.Making modelling, generalization, and justification an explicit focus of instruction can help to make big ideas available to all students at all ages. (Carpenter & Romberg, 2004, p. 5).We face a world that is shaped by increasingly complex, dynamic, and powerful systems of information, such as sophisticated buying, leasing, and loan plans that appear regularly in the media. Being able to interpret and work with such systems involves important mathematical processes that have been under-emphasized in many mathematics curricula. Processes such as constructing, explaining, justifying, predicting, conjecturing, and representing, as well as quantifying, coordinating, and organising data are becoming all the more important for all citizens. Mathematical modelling, which traditionally has been the domain of the secondary school years, provides rich opportunities for students to develop these important processes.A model may be defined as "a system of conceptual frameworks used to construct, interpret, and mathematically describe a situation" (Richardson, 2004, p. viii). By engaging in mathematical modelling students identify the underlying mathematical structure of complex phenomena. Because mathematical models focus on structural characteristics of phenomena (e.g. patterns, interactions, and relationships among elements) rather than surface features (e.g. biological, physical or artistic attributes), they are powerful tools in predicting the behaviour of complex systems (Lesh & Harel, 2003).