Building on research and practice, the National Center on Educational Outcomes (NCEO) has revisited and updated its 2001 document that identified principles and characteristics that underlie inclusive assessment and accountability systems. This report on a principled approach to accountability assessments for students with disabilities reflects lessons learned during the past seven years, presenting six core principles: (1) All students are included in ways that hold schools accountable for their learning; (2) Assessments allow all students to show their knowledge and skills on the same content; (3) High quality decision making determines how students participate; (4) Public reporting includes the assessment results of all students; (5) Accountability determinations are affected in the same way by all students; and (6) Continuous improvement, monitoring, and training ensure the quality of the overall system. Rationale and specific characteristics for each principle are included. A List of Groups and Individuals who Provided Review and Comment during Development of the 2008 NCEO Principles is appended. (Contains 1 figure and 6 tables.)