Since Emmett Betts first devised a framework of independent, instructional, and frustration reading levels in the 1940s, these levels have played a large role in classroom assessment and instruction. It is important for teachers to have a deep understanding of the research that supports the reading level framework. This article identifies four key assumptions upon which the reading levels are based: (1) decoding accuracy and reading comprehension are closely linked; (2) independent reading requires nearly perfect oral reading accuracy; (3) oral reading performance is a reasonable proxy for silent reading behaviors; and (4) certain levels of decoding and comprehension difficulty cause frustration. For each of these assumptions, the relevant research base are summarized and implications for classroom practice are discussed. (Contains 1 table.)