The capacity of educators to dynamically adjust their thought processes and behaviors in response to the diverse needs and characteristics of students in an unfolding teaching situation is widely acknowledged as a cornerstone of effective teaching. While teachers' instructional adaptations have been documented and studied from various angles, the field lacks a robust theoretical framework that precisely distinguishes yet cohesively integrates teachers' cognitive processes and capabilities of adaptive teaching. Addressing this gap, this dissertation presents the Teacher Adaptive-Cognition Theory (TACT), an innovative theoretical framework that aims at providing a comprehensive and much-needed perspective on the cognitive complexity inherent in adaptive teaching. Formulated through a critical and extensive review of adaptive cognition within and beyond the field of teaching, the TACT model distinguishes among behavioral performance of adaptation, cognitive performance of adaptation, and cognitive readiness for adaptation. It posits that analyses of the cognitive processes and capabilities of adaptive teaching should be systematically contextualized within each other. Furthermore, the dissertation delves into the implications of the TACT model for both research and practical application within and beyond the realm of teacher adaptability. By providing this detailed, integrative framework, this dissertation aims to enhance the conceptual clarity needed to guide research in the study of the crucial skill of teacher adaptability in particular and adaptive expertise in general, as well as to foster the development of such skill. [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.]