For several years NAEP developers and administrators have been interested in creating accessible blocks as a means of improving measurement of student achievement at the lower levels of the NAEP performance continuum, including students with disabilities (SD) and English language learners (ELL). Increased precision at the lower levels represents an important validity issue regarding the use of NAEP as a means of benchmarking and interpreting change in national and state performance over time. The purpose of the current study was to design and field test an accessible block alternative for the grade 4 and grade 8 NAEP math assessments. The study was conducted in two phases. The first phase of the study focused on the development of a set of Item Modification Guidelines and Item Modification Procedures, and concluded with a small pilot (n = 671 per block). The second phase of the study focused on applying the Item Modification Guidelines and Item Modification Procedures to create two accessible blocks at each grade level (grade 4 and grade 8), administering the blocks to nationally representative samples of NAEP participants (n = 3,504 for grade 4; n = 3,608 for grade 8), and evaluating the results of the study. Results indicated that accessible blocks significantly reduced estimates of standard error for students at the lower end of the NAEP performance continuum. In addition, results indicated that students who completed an accessible block were significantly less likely to skip items and significantly more likely to complete each item on the assessment. The Item Modification Guidelines and Item Modification Procedures outlined in this study have been incorporated into the regular NAEP item development and review process.