Electrolytes are often treated as simple components of hydration-substances that are added, lost, and replaced. This framing, while common, is incomplete. Electrolytes function as a foundational regulatory system that shapes fluid distribution, enables signaling, maintains gradients, and supports stability across physiological processes. This book examines electrolytes as infrastructure. It reframes balance as a dynamic condition rather than a static target and explores how variation, distribution, and interaction determine outcomes across conditions and time. Rather than focusing on intake alone, it considers how the system behaves, adapts, and destabilizes under changing demands. Written from a systems perspective, Electrolytes develops a structural understanding of how this regulatory network operates without relying on prescriptive protocols or simplified models. The result is a clearer view of how electrolyte dynamics influence physiological stability.