Growing interest in integrated science, technology, engineering, and mathematics (iSTEM) education has been promoted as one way to increase innovation capacity, support future employment, and enhance learning outcomes in K-12 education throughout the USA. Existing efforts to construct iSTEM curricula have largely focused on finding points of integration among commonly shared disciplinary practices, but these efforts have not explicitly accounted for the distinct epistemologies of the disciplines. In this study, we critically examined the concept of iSTEM by conducting a thematic analysis of K-12 STEM learning standards documents to identify cross-cutting themes among the practices of the various disciplines. We then analyzed these themes using disciplinary epistemologies in order to highlight some promises and perils of an integrated approach to STEM education. We identified eight cross-cutting themes: communicating, investigating, modeling, using tools, working with data, making sense of problems or phenomena, solving problems, and evaluating ideas or solutions. Through our analysis of practices and epistemologies, we discuss the promises of iSTEM, including fewer learning standards, enhanced epistemic fluency, increased diversity and inclusion in STEM, and opportunities to challenge settled and siloed disciplinary knowledge. We also discuss potential perils, which consist of conflation and/or exclusion of various STEM practices and epistemologies. We urge continued examination of iSTEM with an eye toward the epistemic implications.