In this paper, the authors describe the initial stage of developing a framework designed to support out-of-field, less experienced or isolated mathematics and science teachers to make decisions about the use of resources in their teaching. The process highlighted the complexity and extent of the knowledge on which expert teachers draw in making such decisions and thus underscored the enormity of the task of teaching out-of-field. The eventual product, the Science, Technology, Engineering and Mathematics: Critical Appraisal for Teachers (STEMCrAfT) framework has proven useful not only for the target audience, but also as a tool for colleagues who take on a mentoring role. The authors begin with a brief description of teacher knowledge before describing the project and then presenting what was unearthed about expert teachers' thinking and knowledge. [This article was first published in the "Australian Mathematics Teacher," in 2016 (EJ1121360).]