Critical thinking continues to be an educational concern even though many school systems, educators, and academic articles have stressed its importance. To teach critical thinking, teachers need to learn what it is and how it is taught. It is unknown to what extent critical thinking skills are taught and assessed in classrooms. The purpose of this grounded theory project study was to determine to what extent critical thinking is taught and to what extent critical thinking activities are used in high school classrooms. Classroom observations were the primary method of data collection, using a pre-existing instrument and protocol, "Classroom Walkthroughs for Continuous Improvement." Interviews with participating teachers were also conducted. The data were collected and categorized through 2 waves of data collection using a constant comparison method of analysis. Open coding was used to establish the initial categories, while axial coding was used to make connections between the central phenomenon and its relationship with other categories. The emergent grounded theory framed by the data postulates that teaching critical thinking is best accomplished within a curricular framework of challenging assignments where students work together to solve problems, and while their teachers adopt a teaching strategy in which they primarily function as coaches. Recommendations called for the implementation of a project in which teachers are explicitly taught to utilize critical thinking skills before introducing their students to the concept. The implications for positive social change include increasing the critical thinking skills of students and heightening teachers' awareness of critically grounding their instruction. Ultimately, this can lead to innovative and creative thinking by graduates, significantly helping to advance society itself. [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.]