As the Framework for Success in Postsecondary Writing illustrates, writing teachers at all educational levels can no longer ignore multimodality and the challenges that come with incorporating multimodal writing—texts composed using a combination of sound, images, video, etc.—into the classroom (NCTE, Framework). A chief struggle most writing teachers face is how to evaluate the multimodal texts their students produce, texts that are inherently diverse. In answer to the calls of scholars such as Yancey, Herrington, and Moran for research exploring multimodal assessment in situated classroom practice, my dissertation examines what K-16 writing teachers are and should be valuing in multimodal compositions and why. By investigating what practitioners and theorists value in new media texts, we can better align our multimodal assessment theories and practices to support effective instruction and assessment of multimodal writing. My study brings together theory and practice to provide guidance for writing teachers to navigate the challenges of multimodal assessment. My findings culminate in a multimodal assessment heuristic based in “design” that allows for the evaluation of not only the product, but also the situated composing practices of writers (Purdy). First inspired by my analysis of the multimodal assessment scholarship, then validated by both my analysis of the assignment sheets and interviews, my design-based assessment model provides a flexible, theoretically-grounded approach to multimodal assessment that reflects what this study suggests writing teacher-scholars most value in their students’ new media texts. My design-based multimodal assessment model integrates the three primary theoretical orientations that most influence multimodal assessment: multiliteracies/multimodality, rhetoric and composition, and new media. It forefronts the importance of valuing students’ situated composing processes and highlights multimodality’s goal of developing writer’s metacognitive awareness and sense of agency. A design-based approach to multimodal assessment emphasizes materially aware composing practices that introduce students to new composing technologies and the principles of graphic design, while not overemphasizing the technology itself. And it places rhetorical savvy as the primary objective. Finally, a design-based approach to multimodal assessment helps break down the dichotomy between print-based and digital texts, pushing writing teachers to embrace the notion that all texts are multimodal.