This paper addresses the first year of a three-year, longitudinal study which introduces mathematical modeling to young children and provides professional development for their teachers. Four classes of third-graders (8 years of age) and their teachers participated in the first year of the program, which involved several preliminary modeling experiences followed by two comprehensive modeling problems over a span of 6 months. Regular teacher meetings involving preparatory workshops as well as reflective analyses were conducted. Analyses of children's responses (in group situations) to one of the modeling problems shows the spontaneous ways in which children engaged in sense making, problem posing, hypothesizing, and mathematizing (including representing). It is argued that modeling tasks of the present type are powerful vehicles for developing important mathematical ideas and problem-solving processes in the early school years. [For complete proceedings, see ED489632.]