In 2004, "response to intervention" was introduced into federal special education law as a method of identifying children with learning disabilities. But RTI was quickly adopted as a model for overall school improvement because of its focus on providing assistance quickly to struggling students, before any academic deficits have a chance to become entrenched. Now, 12 years later, RTI continues to expand its reach and evolve. The same basic framework is used by many schools and districts to support children's behavioral and social-emotional needs, to find and remediate struggling readers, and to identify students in need of special education. And, when it’s used to do all those things together, it’s often called "multitiered systems of supports," or MTSS. And along with that evolution have come some growing pains and some successes. This special report on RTI—Education Week’s second such report—explores the challenges facing educators as they adopt RTI for new uses, scale it up to more schools and districts, and use it to improve learning for all students.