​Soils are particulate materials; fluids and microorganisms fill the pore space. Salient particle level phenomena inc lude fluid- mineral interaction, mixed fluid phase interaction, bio-mediated geochemical processes, size-dependent particle level forces, particle-shape effects, columnar force transfer along the granular skeleton, and multiple internal temporal and spatial scales. These phenomena manifest into complex macro-scale soil response including inherent non-linear and non-elastic behavior; stress-dependent stiffness, strength and volume change; porous and pervious material, inherent and stress-induced anisotropy, timedependent response, and energy coupling among others. Selected particle- level processes and macroscale behaviors are reviewed.