Teachers use the word assessment to describe any method of gathering information about student learning. Whether it be formative assessment (intended to guide instructional decisions) or summative assessment (a reflection on the entirety of student learning from prior instruction), teachers are constantly working to identify what their students know and how deeply they understand what they are learning. The process of assessing student learning requires teachers to look beyond merely correct and incorrect responses that students have given to different questions and assessment items. If a student makes an error, teachers must discern the severity of the error and what caused the incorrect response. Via vignettes, this article takes a look inside first- and fourth-grade classrooms where teachers demonstrate how to use a research-based structure, known as the Initiative for Developing Mathematical Thinking (IDMT) assessment framework, during instruction to choose tasks that elicit different levels of comprehension. The IDMT assessment framework comprises three levels of assessment and draws on more substantive work in the field of task and assessment from such authors as Hiebert (1986), Webb (2002), de Lange (1999), and Stein and Smith (1998): (1) Level 1: Skill; (2) Level 2: Problem solving and concepts; and (3) Level 3: Reasoning and justification. By using the IDMT assessment framework, teachers can thoroughly determine not only what students know but also how deeply students can reason about or understand the content.