Assessment is at the core of high-quality education. When educators purposefully engage in assessment, the learning experiences and outcomes of all students, especially of students diagnosed with specific learning disabilities (SLDs), are improved. In this chapter, we discuss assessment as unfolding within a framework that includes purposes, processes, and tools of assessment existing in the educational context. We open the chapter with an explanation of this framework. Then, in Part 1, we review the construct of SLD and detail how assessment is used within prominent approaches to diagnosis. In Part 2, we discuss how assessment is used to inform instruction before and after diagnosis. We ground our discussion in a vignette that follows Tess, a student who, at the beginning of third grade, has undiagnosed SLD in the area of reading. We show how educators at Tess's school collaborate in the purposeful use of assessment to inform instruction before and after Tess's diagnosis of SLD, highlighting traditional and innovative assessment techniques along the way. [For the complete volume, "Traditional and Innovative Assessment Techniques for Students with Disabilities. Advances in Special Education. Volume 36," see ED614233.]