We present results of an investigation of preservice secondary mathematics and science teachers' conceptions of project-based instruction (PBI) and their enactments of PBI in apprentice (student) teaching. We evaluated their thinking and implementations within a composite framework based on the work of education researchers. We analyzed survey responses, both qualitatively and statistically, from three cohorts of preservice teachers both before and after apprentice teaching. In addition we interviewed and observed a subset of these future teachers. We found that in general the preservice teachers held superficial views of PBI, as compared to the researcher framework. Participants reported time and curriculum restrictions as major barriers; however, teachers for whom enactment of PBI was presented as an explicit goal, and who were given support toward that end, were more likely to enact authentic implementations, regardless of previous reservations about PBI. Without this additional scaffolding, even teachers with high affinity for PBI were unlikely to implement it authentically.