One of the key requirements of the Common Core State Standards for Reading is that all students must be able to comprehend texts of steadily increasing complexity as they progress through school. By the time they complete the core, students must be able to read and comprehend independently and proficiently the kinds of complex texts commonly found in college and careers. This document makes a research-based case for why the complexity of what students read matters. In brief, while reading demands in college, workforce training programs, and life in general have held steady or increased over the last half century, K-12 texts have actually declined in sophistication, and relatively little attention has been paid to students' ability to read complex texts independently. These conditions have left a serious gap between many high school seniors' reading ability and the reading requirements they will face after graduation. This paper then addresses how text complexity can be measured and made a regular part of instruction. It introduces a three-part model that blends qualitative and quantitative measures of text complexity with reader and task considerations. This paper also includes three annotated examples showing how the model can be used to assess the complexity of various kinds of texts appropriate for different grade levels. A bibliography and a glossary of key terms are included. (Contains 18 figures and 9 footnotes.) [For the main report, see "Common Core State Standards for English Language Arts & Literacy in History/Social Studies, Science, and Technical Subjects" (ED522008). For additional appendixes to the main report, see: (1) "Common Core State Standards for English Language Arts & Literacy in History/Social Studies, Science, and Technical Subjects. Appendix B: Text Exemplars and Sample Performance Tasks" (ED522010); and (2) "Common Core State Standards for English Language Arts & Literacy in History/Social Studies, Science, and Technical Subjects. Appendix C: Samples of Student Writing" (ED522009).]