The purpose of this study was twofold: (1) to determine the efficacy of the Leveled Literacy Intervention program (LLI) in increasing reading achievement for K-2 students and (2) to examine LLI program implementation fidelity. This study evaluated LLI in two U.S. school districts and used a mixed-method design to address the following key research questions: "what progress in literacy do students who receive LLI make compared to students who receive only regular classroom literacy instruction?" and "was LLI implemented with fidelity to the developers' program model?" Five elementary schools in the Tift County School District (TCS) in Tifton, Georgia, and four elementary schools in the Enlarged City School District of Middletown (ECSDM) in Middletown, New York, volunteered to participate in the study. A total of 28 LLI teachers and 125 classroom teachers across both districts participated in this study. The findings of this evaluation indicate that LLI combined with regular classroom instruction can positively impact student literacy achievement to a greater degree than classroom instruction alone for K-2 students who are struggling with reading and writing. ELL and special education students can also benefit from the LLI program, some with strong, educationally significant effects. Robust effects were found on the LLI Benchmarks across all grade levels for students who received LLI. Students in LLI achieved between 1 1/2 benchmark levels up to almost 5 1/2 benchmark levels while students who did not receive LLI achieved between less than 1 benchmark level up to about 3 benchmark levels. Students in LLI also finished on par with grade-level goals. Further, because the majority of students in the study were economically disadvantaged, the findings indicate that the LLI program is effective with this high-risk population. However, the current study is limited in generalizability to rural and suburban populations, as well as those economically disadvantaged. The efficacy of the LLI program has important implications for schools and districts with limited resources and time available for early reading interventions. LLI's short-term, small-group format allows a greater number of struggling students to achieve grade-level competency within a shorter period of time. LLI's success with early learners also demonstrates its potential for reducing the development of chronic, long-term reading deficiencies and academic problems. Given the positive potential of the LLI program, future research is warranted in this area. Specifically, additional studies should be conducted in urban areas, and longitudinal tracking should be utilized to determine the long-term impact of LLI on students' literacy development. (Contains 13 tables.)