Human systems integration (HSI) acknowledges that the human is a critical component in any complex system (Booher, 2003). It is an interdisciplinary approach that makes explicit the underlying tradeoffs among a set of technical domains, facilitating optimization of total system performance in both materiel and nonmateriel solutions to address the capability needs of organizations. HSI is deeply rooted in the military-industrial complex. Whether looking at HSI in the United States and Canada or at its British counterpart, human factors integration, one can trace the inception of this interdisciplinary field to governmental efforts to manage defense acquisitions more effectively. Many of the chapters in this handbook address HSI in the private sector, and it is evident that steady progress is being made to apply HSI in civilian and commercial contexts. However, the focus of this chapter is on how HSI is practiced in the U.S. military, particularly in the U.S. Army, and primarily in the context of the U.S. Department of Defense (DoD) acquisition life cycle. Here, we review the domains of HSI and HSI tradeoffs, illustrating how trade-offs are made using actual examples of HSI in action. We also describe how HSI is typically implemented at various stages of the DoD acquisition life cycle.Like all interdisciplinary fields, HSI overlaps with other closely related specialties. HSI has much in common with human factors engineering (HFE) and systems engineering (SE). This chapter explores both the commonalities and differences among these disciplines, looking at examples of why other disciplines do not address HSI in the most effective manner.This chapter was authored by employees of the United States government as part of official duty and is considered to be in the public domain. Any views expressed herein do not necessarily represent the views of the United States government, and the authors' participation in the work is not meant to serve as an official endorsement.