Indigenous-language immersion (ILI) is a form of schooling where all, or nearly all, classroom instruction in every subject area is conducted in an Indigenous language. This dissertation comprises three case study comparisons of neighboring pairs of ILI and English-medium school programs. The first case study examines two elementary schools in the same community. The second case study consists of two independent colocated schools serving elementary and intermediate grades. The third case study compares the ILI and English-medium programs at an intermediate school serving 6th-8th grades. Various academic achievement measures, including English language arts and math standardized assessment scores, are examined to quantify the contrasting associations between ILI versus English-medium instruction and student outcomes, after accounting for observed student background characteristics. On mainstream English-language measures of academic achievement, we find that with few exceptions, ILI students at the case study sites generally scored as high as, or higher than, their Indigenous peers who experienced English-medium instruction. At the same time, when assessed on their Indigenous language proficiency, the ILI students demonstrated consistent maintenance and growth across various Indigenous-language proficiency domains. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]