This book redefines concrete as a moisture-responsive, porous material, revealing how cement-based materials interact with water through their nanoscale pore structure. This transforms our understanding of dimensional stability, long-term durability, and other aspects of concrete science.Using low-field nuclear magnetic resonance relaxometry, the author non-destructively characterizes the in-situ nanoscale pore structure of white cement-based materials and uncovers how it evolves with water content. The author identifies "water sensitivity" - the wetting expansion and drying contraction of calcium silicate hydrate - as the mechanism behind this behavior, analogous to clay minerals. The water sensitivity theory is validated by the microstructural characteristics of the dynamic pore structure, including a layered structure with a large specific surface area and a negative surface charge, and is substantiated by experimental results on permeability and sorptivity. Building on this framework, the book reexamines the effects of the water-to-cement ratio and curing temperature on the pore structure and water permeability of cement mortars. The book also successfully models phenomena, including deviated long-term water absorption, the anomalous dependence of sorptivity on initial saturation, and the isothermal drying of cement-based materials.This book will serve as an essential reference for concrete scientists, materials engineers, and researchers studying the dimensional stability and long-term durability of cement-based materials, as well as their many other moisture-regulated performances.