The design, flexibility, and iterative nature that is inherent to Universal Design for Learning (UDL) makes it difficult to consistently measure. With federal policy encouraging the implementation of UDL, there is an increased need for practitioners to reliably measure the occurrence of UDL. The UDL Observation Measurement Tool (UDL-OMT) was developed to measure UDL implementation in classrooms. This article presents the conceptual underpinnings of UDL measurement and the results of an initial field test. Results indicate that the UDL-OMT has good to excellent internal consistency and can characterize differences in UDL implementation across a continuum of settings. Discussion focuses on the reliability of the UDL-OMT and its potential as a formative evaluation tool for practitioners and school-based personnel. Additional considerations include promising research applications and how the nature and context of classroom instructional factors as well as observers' UDL knowledge influence interpretations of observations.