Teachers need to develop the ability to adapt curriculum materials. Two elementary teachers, Maggie and Catie, were asked to write narratives about their use of and changes to particular reform-oriented science lesson plans. Maggie drew on her knowledge of and experiences with students, as well as other knowledge, experiences, and resources, to make productive changes to account for her students' prior knowledge and abilities. Catie based her curricular adaptations on her learning goals--but these were not aligned with the learning goals of the curriculum materials. The paper discusses implications for teacher education, professional development, and educative curriculum materials.