Previous research has provided insight into teachers’ adaptations during instruction (Duffy et al., 2008), yet it has not been determined what knowledge teachers draw upon to make their decisions. Therefore, this study examines the knowledge teachers draw upon to make adaptations during instruction. Two case studies were conducted with first-grade teachers during science and social studies instruction. I observed to collect the adaptations the teachers made during instruction and in post-lesson interviews I asked the teachers to confirm the adaptations, to give rationales for why they adapted and to explain the knowledge they drew upon to make the adaptation. Adaptations, rationales, and knowledge categories were coded and analyzed to determine the categories of knowledge teachers relied upon to make adaptations during lessons. I found the categories of adaptations, rationales, and knowledge the two teachers used were related to one another, but not in the way that I had anticipated. The teachers used categories of adaptations, rationales and knowledge that seem to put students at the foundation of thoughtfully adaptive research.