District leaders in a large urban school district in central Florida wanted to test the efficacy of a new curriculum designed to enhance the word knowledge of grade 5 students so as to improve reading achievement. The new curriculum, called Word Knowledge Instruction (WKI), consists of 15-minute lessons taught 4 days a week for 20 weeks. The lessons address state standards and cover 20 prefixes and suffixes. Forty-two schools participated in the study, with 92 English language arts (ELA) teachers in high poverty schools randomly assigned within schools either to use WKI or to continue using their regular ELA instructional materials. Key findings from the study include: (1) WKI had a significant, positive impact on students' ability to correctly extract and spell a base word from a derived word compared with students in the business-as-usual control group; (2) WKI had no impact on any other short-term or long-term outcome measure; (3) The impact of WKI on short-term and long-term outcome measures was comparable for English learner students and non-English learner students; and (4) The key implication is that transferring taught skills to related but not directly taught skills might require more intense or longer duration instruction. [For the full report, see ED611683. For the appendix, see ED611685.]