The Cognitively Based Assessment of, for, and as Learning ("CBAL"™) research initiative is aimed at developing an innovative approach to K-12 assessment based on cognitive competency models. Because the choice of scoring and equating approaches depends on test dimensionality, the dimensional structure of CBAL tests must be understood. The purpose of this study was to investigate the dimensionality of 4 CBAL Grade 8 writing tests. Each of the 4 tests focused on one of the following writing genres: persuasive letter/memorandum, research-based report (pamphlet), persuasive essay, and literature essay. Dimensionality was investigated using exploratory and confirmatory factor analyses. The results show multidimensionality and support subscore structures and bifactor task models for all 4 tests.