This project is grounded in a theoretical framework that specifies key epistemological features of scientific experimentation. On the basis of this framework, the project will develop a computer-based environment in which seventh graders can conduct simulated experiments in the domain of cell biology. The simulated experiments will be designed to capture many of the crucial epistemological features of authentic scientific experimentation. The computer-based experimentation environment will then be used in a series of studies to investigate how students learn complex forms of scientific reasoning. A main focus of the studies will be to investigate the sociocognitive processes by which computer simulations promote complex learning during classroom instruction. The planned studies include two large classroom experiments that will provide a rich, detailed picture of how seventh graders' reasoning and knowledge of cells change, moment to moment, during classroom instruction. The classroom instruction, using an instructional method called Collaborative Reasoning about Evidence, will interleave whole-class discussions with small-group work in which students conduct and discuss simulated experiments. The studies will aim to provide a fine-grained analysis of how whole-class discussions and group interactions mediate students' learning. In sum, this program of research is intended (a) to provide new insights into how to teach students to reason about epistemologically authentic experimentation and (b) to produce tested instructional software, materials, and methods that teachers can readily access and use in their classrooms.