The Individualizing Student Instruction (ISI) classroom observation and coding system is designed to provide a detailed picture of the classroom environment at the level of the individual student. Using a multidimensional conceptualization of the classroom environment, foundational elements (teacher warmth and responsiveness to students, classroom management) and instructional elements (teacher-child interactions, context, and content) are described. The authors have used the ISI system to document that children who share the same classroom have very different learning opportunities, that instruction occurs through interactions among teachers and students, and that the effect of this instruction depends on children's language and literacy skills. This means that what is effective for one child may be ineffective for another with different skills. With improving classroom observation systems, the dynamics of the complex classroom environment as it affects student learning can be better understood. (Contains 6 figures and 2 tables.)