The proposed research will study what and how middle-school children learn about complex systems, the lesson plans used by middle-school teachers, and the mental models children develop as a result of classroom activities. The main learning goal is to promote the acquisition of a deep understanding of the nature of complex physical and biological systems at multiple levels of abstraction and aggregation by middle-school children. In particular, the proposed project will explore the use of structure-behavior-function (SBF) models in the establishment and maintenance of classroom fish-aquarium systems as the context for learning about complex systems. The main technology goal is to design, develop and evaluate an interactive environment called ACT built around an interactive SBF modeling tool for use in middle-school classrooms. The project expects the children to use the ACT environment in conjunction with physical aquaria and existing NetLogo simulations of aquaria. Instead of prescribing a specific method for using the ACT environment, there will be support for multiple lesson plans. The evaluation criteria for the learning goal will be the depth and accuracy of the SBF models developed by the children, and the effectiveness of the SBF models in the establishment and maintenance of classroom aquaria. The usefulness of the ACT environment for these tasks will be assessed in middle schools in central New Jersey, which have very diverse demographics.