This study explored Resident Scientists’ curriculum, instructional and assessment decisions for high school classrooms. We also attempted to find out the factors that influenced their decisions. A sample comprised eight Resident Scientists who were providing science content knowledge and pedagogical support to teachers in schools through the National Science Foundation (NSF) GK-12 outreach project in the Midwest of the USA. Data were collected through an open-ended questionnaire. Results show that Resident Scientists made short-termed decisions on curriculum, instruction and assessment for high school classrooms. Their decisions were mainly influenced by partner teachers, textbooks, and antecedent factors. Moreover, their decisions encompassed elements of accountability in the school system. They also exhibited lack of knowledge about the USA Science Education Standards. However, their decisions were within the framework that integrated content and pedagogical support to teachers. The findings have implications for science teaching and learning and the NSF GK-12 program.