This paper presents the results of a pre-test, instruction, post-test study that investigated students' integer mental models and how their models changed based on instruction. Sixty-one first graders' responses to questions about the values and order of negative numbers were categorized according to a series of mental models. The models reveal that initially, students over-rely on the values and/or order of whole numbers to varying degrees when working with negative numbers. Focused exploration on the properties of negative numbers helped students transition to more sophisticated mental models compared to only learning about moving in positive and negative directions on number lines. [This research was funded by a Stanford University School of Education Dissertation Grant. For the complete proceedings, see ED584829.]