Educators and instructional designers have used the Universal Design for Learning (UDL) framework to guide their design of inclusive instruction for students with and without disabilities. Despite UDL having entered its 4th decade of development and research, there have been ongoing critiques of UDL for lacking clarity in definition, challenges with implementation, and insufficient evidence of its effectiveness. These critiques warrant further evaluation of UDL, especially with a focus on the theoretical underpinnings behind its conceptualization and implementation. Thus, we synthesized 32 peer-reviewed studies published between 1999 and 2023 that focused on UDL implementation in preK-12 educational settings and measured various aspects of student learning outcomes (e.g., cognitive, motivational, and behavioral). Specifically, we evaluated each study's intervention or instructional design in terms of its alignment to UDL checkpoints, guidelines, and/or principles as well as existing theories of learning or instructional design. Results revealed several interrelated challenges that stymie UDL research, including the absence of explicit alignment between UDL checkpoints and intervention or instructional designs investigated in the extant literature, the uneven coverage of implemented checkpoints and corresponding guidelines, the overlap among multiple checkpoints and guidelines, and the lack of theoretical guidance regarding the design and implementation processes. Based on these findings, we provide recommendations for strengthening the research base for less frequently applied UDL checkpoints, recommendations for documenting checkpoints and relationships among checkpoints as indispensable components of UDL implementation, and directions for future research conducted via systematic UDL implementation guided by established theories.