Understanding the fundamental mechanisms of fluid flows and reactive transport in natural systems is a major challenge for several fields of Earth sciences (e.g. hydrology, soil science, volcanology) and Geo/Environmental-engineering (CO2 sequestration, NAPLS contamination, geothermal energy, oil&gas reservoir exploitation). The hierarchical structures of natural system (e.g. heterogeneity of geological formations) as well as the different behaviour of single and multi-phase fluids at the pore-scale coupled with the nonlinearity of underlying reactive processes necessitate investigating these aspects at the scale at which they physically occur, the scale of pore and fractures. Recent improvements in pore-scale computational modelling, together with the development of non- invasive microscopic imaging technology and the latest microfluidics technics are allowing the vast field of porous & fractured media research to benefit of major advances due to: 1) an improved understanding and description of pore-scale mechanisms and 2) the ability of thinking in terms of coupled processes. The contributions collected in this Special Issue, although far from constituting a comprehensive picture of the “pore-scale world”, however offer a good example of the potentialities of such an approach to investigate a wide range of processes usually observed at macro-scale, but whose underlying physical and chemical processes take place at micro-scale.