The National Accessible Reading Assessment Projects (NARAP) is a collaboration of two projects funded by the U.S. Department of Education to conduct research and development on accessible reading assessments for students with disabilities that affect reading. The goal of these projects is to produce research findings and assessment techniques that demonstrate how large-scale assessments of reading proficiency can become more accessible and valid for all students, while also meeting the assessment requirements of the No Child Left Behind Act of 2001 (NCLB). Understanding the need for accessible assessments is key to determining the steps that must be taken to develop such assessments. One of the first steps in designing accessible assessments is the precise definition of constructs being measured. Such definitions permit construct-relevant factors to be distinguished from construct-irrelevant factors, thus facilitating the removal of barriers to accessibility while maintaining valid measurement of the constructs of interest. Similarly, distinguishing various components of the construct may facilitate development of assessments that can reveal strengths in some components that would otherwise be masked by weaknesses in other components. Thus, one of NARAP's first tasks was to develop a definition of "reading proficiency." In order to accomplish this, NARAP collected information in a variety of ways, including: (1) reviewing existing definitions of reading proficiency (e.g., various reports about reading including the NRP, RAND, PISA, PIRLS); (2) convening a panel of experts to provide input; and (3) conducting focus groups. This paper is a synthesis of the information collected and describes a set of principles and key unresolved issues described under each principle. These principles and issues will guide the next phases of the project (research and development). This paper does not focus on the assessment itself; rather, the assessment developed will be informed by the research and then addressed in a separate paper focusing on the principles and guidelines of accessible assessments. (Contains 1 footnote.) [This report was prepared by the National Accessible Reading Assessment Project's Designing Accessible Reading Assessments (DARA) (based at the Educational Test Service (ETS)); and Partnership for Accessible Reading Assessment (PARA) (a consortium of the National Center of Educational Outcomes (NCEO), the National Center for Research on Evaluation, Standards, and Student Testing (CRESST), and Westat).]