In this article, the author presents three different tasks that can be used to assess students' understanding of the concept of length. Three important measurement concepts for students to understand are transitive reasoning, use of identical units, and iteration. In any teaching and learning process it is important to acknowledge students' existing knowledge and focus teaching on moving students on from this point. Assessment tasks that provide insight into students' current understanding of, or misconceptions about, these concepts is, therefore, vitally important. The insights gained from these tasks can be used to inform future teaching and learning. (Contains 1 figure.)