Aqueous fluids migrating in the Earth’s crust and shallow subsurface potentially play an important role for drinking-water supply, the formation and transport of exploitable reservoirs, or geothermal energy use. Fluid transport, carrying dissolved and colloidal matter, may occur diffusive through the interconnected pore space porous flow as well as focused through fissures and fault zones. Whereas flow processes acting on individual scales are relatively well understood, feedbacks between scales are still major open research questions, which can be only addressed with combined methodologies. Therefore, we welcome especially multidisciplinary studies from all fields of geosciences, which contribute to better understanding of multi-scale fluid-systems.