Although millions of dollars and weeks of instructional time are spent nationally on testing students, educators often have little information on how to choose appropriate assessments of adolescent reading for informing instruction. This guide is designed to meet that need, by drawing together evidence about nine of the most commonly-used, commercially-available reading comprehension assessments and providing a critical view into the strengths and weaknesses of each. In so doing, the authors focus on the utility of assessments for the purposes of screening groups of students to identify those who struggle and diagnosing the specific needs of students who struggle. Motivated primarily by the many questions that the authors receive from principals, literacy coaches, and district curriculum leaders about diagnostic assessment for students in grades four through twelve, this guide aims to provide those decision-makers with the tools they need to make informed decisions. Note on Methodology is appended. (Contains 11 tables and 7 endnotes.) [For related reports, see "Adolescent Literacy Programs: Costs of Implementation. Final Report from Carnegie Corporation of New York's Council on Advancing Adolescent Literacy" (ED535296); "Time to Act: An Agenda for Advancing Adolescent Literacy for College and Career Success. Final Report from Carnegie Corporation of New York's Council on Advancing Adolescent Literacy" (ED535318); "Adolescent Literacy and Textbooks: An Annotated Bibliography. Final Report from Carnegie Corporation of New York's Council on Advancing Adolescent Literacy" (ED535323); "Reading in the Disciplines: The Challenges of Adolescent Literacy. Final Report from Carnegie Corporation of New York's Council on Advancing Adolescent Literacy" (ED535297); and "Adolescent Literacy Development in Out-of-School Time: A Practitioner's Guide. Final Report from Carnegie Corporation of New York's Council on Advancing Adolescent Literacy" (ED535304).]