Scientists, funders, and policy makers agree that there are challenges confronting humanity that manifest globally, operate at multiple scales simultaneously, and exhibit complexity that defies treatment from isolated perspectives. To adequately respond to these complex, global challenges, which include climate variability, food security, and sustainable energy, we must understand their complexity. The kind of complexity exhibited by global challenges involves, at least in part, the unified combination of different elements that interact in complex ways. Understanding such a challenge is typically taken to require research involving different types of expertise, each focused on a different element of the challenge, that are combined, or integrated, into a unified account. Integration of this sort – which can be called cross-disciplinary integration (CDI) to highlight the centrality of expertise from across the academic disciplines – is widely deployed by those who investigate global challenges, but there is no systematic account of it, leaving it under-theorized and poorly understood. This lack of theoretical development undermines our understanding of global challenges and our ability to respond to them. This project focuses on developing a systematic, detailed account of CDI in scientific discourse and evaluating it across a wide range of contexts in which different disciplinary perspectives are combined, i.e., cross-disciplinary contexts. This account will expand an input-process-output (IPO) model of CDI first proposed and successfully piloted in previous work. After grounding the model in a more general understanding of IPO approaches in psychology and elsewhere, the project will proceed with a review of literatures in which CDI is used to explain cross-disciplinary phenomena and in which it is theoretically modeled. The IPO model will then be developed and calibrated in light of what is learned in the literature reviews, yielding a version of the model that explains CDI across a range of contexts and provides a theoretical foundation for empirically rigorous identification, explanation, and evaluation of integrative phenomena in cross-disciplinary research. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.