Even though textbooks are a central component of the daily instructional practice of English language teachers, relatively little research has been conducted on how teachers actually use (i.e., adapt) textbooks in the classroom. This gap is aggravated by the fact that the terminology proposed in the literature to analyse teachers' textbook use is characterised by inconsistencies because different terms denote the same adaptation techniques, identical terms refer to different techniques and suggested frameworks differ in the fact that comparable techniques are allocated to different categories. This inconsistency mirrors the difficulty of operationally defining adaptation techniques, as the terms used may be unambiguous but vague and therefore of reduced explanatory power or more specific but potentially unreliable because an adaptation may be matched to different terms given the complexity of a particular textbook adaptation. Discussing these aspects, this paper proposes a research-informed framework to contribute to a systematic description of textbook adaptation in foreign and second language teaching. Examining adaptation as a process, it is argued that teachers, driven by an identified or felt mismatch between the textbook and other factors (e.g., school facilities, the learners, teacher cognition, course requirements, or outdatedness of the materials), engage in adaptation based on principles (i.e., ideas about best practices, by making changes to the content, the language and/or the sequence of activities offered by the textbook authors). Even though related to English language teaching, this paper does not exclusively inform this context as it offers implications for research on textbook use in other disciplines.