Purpose of Review <p>Integrated Transmission and Distribution (ITD) systems for electric power are complex systems encompassing strongly coupled physical, economic, and legal processes. This review identifies and assesses design strategies for ITD systems that permit this complexity to be systematically addressed. </p> Recent Findings <p>A nine-tiered Design Readiness Level (DRL) classification is used to sort an illustrative collection of recent ITD system design studies into readiness tiers ranging from conceptualization to real-world deployment. Computational platforms are seen to provide key support for traversing the “Valley of Death” tiers separating typical university design research from typical design research carried out at industrial and government facilities. </p> Summary <p>Agent-based co-simulation platforms, enhanced by developments in data-assisted generative artificial intelligence, permit reduced computational complexity, flexible tailoring of model simplifications to purposes at hand, matching of modeled agents to empirical referents, and systematic testing of design aspects that involve coupled physical, economic, and legal processes.</p>