Computational modeling has been shown to support mechanistic reasoning in science education. Many studies use non-computational modalities, such as written responses and utterances, to capture and make claims about students’ mechanistic reasoning during computational modeling units. However, unlike computational models, these modalities of expression are less strictly aligned with the details of mechanistic reasoning (MR) and, therefore, may not accurately reflect students’ full reasoning. Understanding how students express their mechanistic reasoning across modalities is important for researchers to gain fuller insights into students’ developing MR. In this chapter, we inspect five elements of mechanistic reasoning to examine how they are expressed throughout a 6th-grade computational modeling science unit and how these expressions shift over the course of the lesson and across different modalities of student work (e.g., utterances, drawings, written responses). Certain aspects of MR, such as describing the organization or properties of system entities, were more difficult than others across modalities. While students’ MR develops over the course of the unit, as expected, many offered fuller expressions of MR during a verbal presentation of their code rather than on a written post-test at the end of the unit.