The purpose of this chapter is to describe tier 3 reading interventions used for primary grade students (in kindergarten through third grade). Within multi-tier response-to-intervention systems, tier 3 is reserved for the very few students with the most persistent reading difficulties, having demonstrated significant difficulties in learning to read even with tier 1 and tier 2 supports. As explained in greater detail, tier 3 may be a last layer of intervention in general education that serves as part of a prereferral process or it may be special education. Following the description of the conceptual framework for the chapter, and the discussion of variability in tier 3 implementation, the literature is reviewed to learn whether it is known what is necessary and sufficient to help students who are struggling to read, including students with reading or mild-to-moderate intellectual disabilities. Implications for research are discussed and directions for future research are identified.