The study started as a critical response to a new sheltered science content course introduced by the school district where I teach ESOL students. Although the course was a bold and timely initiative, it was not supported by a well-built curriculum, realistic educational goals, and appropriate instructional materials. As I was unsatisfied with what I was doing at my workplace, I embarked on the road of self-reflective, practitioner action research grounded in cultural-historical activity theory (CHAT) in order to elaborate an alternative approach for how to teach science content and English to high school students using native language support. The following research questions guided my inquiry: (1) How can I, as an ESOL teacher, effectively use native language support in a particular high school freshmen ESOL science class in order to help students master both content and language? (2) What instructional activities could be useful to promote students' mastering content and language? (3) What measuring techniques could be applied to monitor students' progress? In my study I effectively complimented traditional qualitative and quantitative action research methods with novel instrumentality of CHAT theory, such as a model of expansive learning and a model of the human activity system. During my study I (a) developed instructional methods and materials for the ESOL science content course, (b) examined the process of their development by means of self-reflection, and (c) evaluated the benefits of these methods and materials for students' learning. The findings of the study displayed that four instructional activities with native language support, such as (1) bilingual dictionary activity, (2) vocabulary development activity, (3) functional grammar activity, and (4) translation practice activity were beneficial for students' mastering English and science content. The findings of the study also demonstrated how the process of teacher learning and improvement of teacher practice develop. [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.]