The purpose of this study was to examine the extent to which three teachers' professional experience and existing orientations toward teaching and learning mathematics and science influenced their implementation of a project-based curriculum (i.e. project-based learning (PBL)). Data sources included interviews, videotapes of classroom activity, and a teaching philosophy questionnaire. Data analysis was conducted using an iterative coding technique. We found coherency between the teachers' perceived orientations and their emergent ideas about PBL, their sense of PBL as being compatible with their goals, and the specific challenges with which they struggled. All the teachers wanted their students to be successful; however, different definitions of success led to quite different approaches toward teaching, and for the most part, these differences appeared to have occurred because of existing orientations the teachers held for teaching their discipline. Implications for professional development taking into account teachers' orientations and thus their professional experience are discussed, as well as disciplinary challenges to using PBL.