This article describes a process called "Backing Up" which is a way to preassess student understanding of a topic and gauge student readiness to move forward in the learning process. This process of backing up begins with using responses to a word problem to identify categories of students' understandings in relation to the expectations of the standard and using this information to make instructional decisions. In some instances, students will provide evidence of meeting or exceeding lesson expectations; instructional decisions, therefore, will need to advance their thinking. Instructional decisions for other students, who are working toward lesson expectations, should help them connect prior knowledge to new concepts. Students who are lacking fundamental understandings require instruction aimed at filling gaps in prior knowledge. This article demonstrates this backing-up process--by examining categories of student work taken from a carefully crafted word problem--and suggesting instructional decisions. This assessment-driven process for making instructional decisions is crucial in advancing students' understanding of fractions. By starting with a carefully crafted problem, teachers are able to identify student understandings and misconceptions and make instructional choices by which teachers can could guide students to their goal.