The purpose of this action research was to evaluate how the implementation of asynchronous online discussions into the classroom impacts the critical thinking skills of secondary language arts students. Although critical thinking has shown to be an important 21st century skill (Paul & Elder, 2002), many students are leaving high school lacking the thinking skills necessary to succeed in a modern society (Arum & Roksa, 2011; Conley, 2015). Standardized testing results within the population of this study indicated similar critical thinking deficits as the rest of the nation. This study focused on two central research questions: (1) To what extent will the integration of asynchronous online discussions into the English curriculum impact the critical thinking skills of language arts students? (2) How do language arts students perceive the use of asynchronous online discussions to impact their critical thinking skills? In this convergent parallel mixed methods study, students enrolled in my English 1 Honors course (n = 46) engaged in asynchronous discussion which was centered on a structured weekly debate. Debate groups received increasingly complex prompts each week and participated in required periods of reading, reflecting, writing, and responding. The Cornell Critical Thinking Test was used as a pre- and post-test to measure the impact of the intervention on student critical thinking skills. In addition, student discussion posts were analyzed at the beginning, middle, and end of the intervention with the Holistic Critical Thinking Scoring Rubric to examine how critical thinking skills evolved throughout the course of the study. Students completed a Likert style questionnaire, and a subset of students participated in focus group interviews about their perceptions of how asynchronous discussion impacted their ability to critically think. Transcripts of the focus group interviews were coded looking for patterns and similar ideas in order to generate general themes about the data. Results from all four measures indicated that asynchronous discussions positively impacted student critical thinking skills. Likewise, students had positive perceptions of asynchronous discussions and its impact on their critical thinking ability. Implications on developing secondary school students' critical thinking are discussed. [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.]