Appendix A of the Common Core State Standards (hereafter CCSS) contains a review of the research stressing the importance of being able to read complex text for success in college and career. The research shows that while the complexity of reading demands for college, career, and citizenship have held steady or risen over the past half century, the complexity of texts students are exposed to has steadily decreased in that same interval. In order to address this gap, the CCSS emphasize increasing the complexity of texts students read as a key element in improving reading comprehension. The importance of text complexity to student success had been known for many years prior to the release of the CCSS, but its release spurred subsequent research that holds implications for how the CCSS define and measure text complexity. As a result of new research on the quantitative dimensions of text complexity called for at the time of the standards' release, this report expands upon the three-part model outlined in Appendix A of the CCSS in ELA/Literacy that blends quantitative and qualitative measures of text complexity with reader and task considerations. It also presents new field-tested tools for helping educators assess the qualitative features of text complexity. [For "Common Core State Standards for English Language Arts & Literacy in History/Social Studies, Science, and Technical Subjects. Appendix A: Research Supporting Key Elements of the Standards, Glossary of Key Terms," see ED522007.]