Volume 22, Number 4, 2004 Contents: Introduction to the Special Issue: Investigating Participant Structures in the Context of Science Instruction. I. Tabak, E. Baumgartner, The Teacher as Partner: Exploring Participant Structures, Symmetry, and Identity Work in Scaffolding. J.L. Polman, Dialogic Activity Structures for Project-Based Learning Environments. L.L. Cornelius, L.R. Herrenkohl, Power in the Classroom: How the Classroom Environment Shapes Students' Relationships With Each Other and With Concepts.