This paper presents the design and initial learning research with the MMM modeling platform, seeking to advance middle school students' learning through constructing computational models of complex physical and chemical systems. A complexity-based structure of an MMM interface is introduced. It suggests that a complex system can be described and modeled by defining entities, their actions, interactions with each other, and interactions with their environment. MMM applies to a wide range of phenomena, targeting learning transfer and generalization. Design principles of MMM are presented and discussed based on a study with seventh-grade students. The study is a quasi-experimental, pretest-intervention-posttest control-comparison-group design. Findings from a quantitative analysis of the questionnaires show that engaging students with the construction of models using MMM significantly promoted students' conceptual learning and enhanced their systems' thinking compared with a comparison group who followed a normative curriculum. Students' responses to the worksheets showed mutual effects between improving the practice of modeling and promoting conceptual understanding and systems thinking. A qualitative analysis of screen-capture movies of one pair of students and their log files revealed that, in a later construction activity, their constructed models grew in sophistication and they articulated their thinking and learning in depth, using more sophisticated relationships between concepts.