This dissertation presents the first Systemic Functional Linguistics-based analysis of the teaching/learning of computational thinking through computer programming and comprehensive analysis of discourse of a whole computer programming course at any educational level. The current educational research raises questions about the nature of authentic computational thinking teaching/learning environments and how they happen moment-to-moment. In one such environment, I examined the discourse of a facilitator, three students, and their Language Arts teacher in an introductory middle school after-school course (approximately 30 hours) in spring 2017 as students created a video in Python. Methodologically, I show how a Systemic Functional Linguistics-based analytical framework can operationalize the dimensions of an authentic bilingual (English-Spanish) computer programming environment, student positioning and indicators of computational thinking learning. I identify the following dimensions: complexity (abstraction included), pragmatism, procedurality, dependency, and flexibility. The facilitator positioned the students as capable computational thinkers and computer programmers whose prior world experience and linguistic identity mattered. She also positioned them to collaboratively model their prototypes with grade-level mathematics; create the algorithm; communicate algorithm thinking and computational thinking. I identify relevant teaching strategies; indicators of student learning were found. Strategies include: (1) drawing on the students' languages and cultural resources, (2) capitalizing on student-known mathematical concepts, (3) using a soft focus on concepts, and (4) adopting a motivational, pragmatic, mathematics-based heuristic procedure. My findings illuminate the nature of authentic computational thinking environments and suggest teaching practices that prioritize student creation and communication of meaningful, simple algorithms and programs over complex conceptual explanations. [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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]