Complex Systems Engineering: Theory and Practice represents state-of-the-art thought leadership on system complexity for aerospace and aviation, where breakthrough paradigms and strategies are sorely needed. The costs and consequences of current knowledge and practice gaps are substantial. In short, this problem is caused by several factors: the lack of human capacity to comprehend complexity without machine/autonomation interfaces, the rapid pace of changes in the sector, and the increasing complexity and complicatedness of systems of all types and sizes (occurring by design and by default). The chapters in this volume are derived from the work of the noted Complex Aerospace Systems Exchange (CASE) Scholars, who presented their work at the AIAA SPACE Forums in 2016 and 2017. The CASE Scholars program was begun to support the needs of practitioners facing perplexing systems challenges through the work of outstanding systems academics from engineering and the social sciences. In addition, the CASE Scholars program enabled CASE to better capture and document the well-informed intersection of users and theorists regarding system complexity aerospace/aviation challenges and possibilities. The breadth of topics was selected to provide an enriched view of all types of systems-technical, machine, and human systems-to both practitioners and academics. There are many sides to every system, and this volume attempts to challenge the critical process.