This article explores the decisions that science teachers make when they plan for instruction. It is a case study analysis of five teachers in a high school and probes into the personal reasons, beliefs, and dilemmas underlying their decisions. These decisions, while serving many purposes, had a common structure which involved tradeoffs and compromises. The decision represented the end result of the conflict between a cluster of teacher intentions and a melange of ideas about student characteristics. Teachers appeared to make decisions within a framework that holistically integrated science content and practical classroom knowledge-a knowledge system that includes the basic beliefs of a teacher and the socialization of students. By understanding how and why teachers tend to make their decisions, one gains practical insights into the act of teaching science. These insights are of particular interest to science curriculum specialists who wish to understand how teachers use science curriculum materials for the purpose of socializing students.