Tiered instruction is grouping students for instruction based on their prior background knowledge in a given subject area. In this study, students were either in a control secondary science classroom or a classroom in which instruction was tiered. The tiered instruction was designed to match high, middle, or low levels of background knowledge on astronomy and Newtonian physics. The seven control classrooms received middle-level nontiered instruction, whereas the seven treatment classrooms delivered three levels of tiered instruction. The results of this study showed a significant difference between the scores of low background knowledge learners who received tiered instruction and low background learners who did not receive tiered instruction, indicating that tiered instruction may be especially beneficial for lower level learners. Through the implementation of this study, the researchers found that: (1) professional support for teachers is critical to the success of tiered instruction; (2) a strong background in the subject matter and a thorough understanding of the range of potential learning activities appropriate to the targeted levels of learners is essential; and (3) the implementation of a change of instructional and classroom organization, pedagogy, and expectations needs to be systematically introduced over time. (Contains 2 figures and 10 tables.)