This open access book presents the collective research outcomes of the German Research Foundation (DFG) funded Priority Program (SPP 2005): Opus Fluidum Futurum-Rheology of reactive, multiscale, multiphase construction materials, a multidisciplinary initiative to advance the science of rheology-based sustainable construction processes. This book delivers comprehensive insights into: (a) Interactions of reactive particles, with a focus on how particles of various reactivity, sizes, and shapes engage in physical and chemical interactions. (b) Deformation behavior of fresh concrete with new insights into the rheological assessment of cementitious mixes containing large aggregates and fibers. (c) Concrete transport and processing, examining how concrete flows during pumping, filling formwork, and placement using advanced techniques such as 3D printing. All insights are supported by carefully designed experiments and simulations spanning meso- to macro-scales, laying the groundwork for innovative materials and processes that address the evolving needs of modern construction.