Constraining the rheological behavior and slip dynamics of faults is of fundamental importance for the understanding of plate tectonics as well as earthquake generation. This task, however, is exceedingly difficult as lithosphere‐scale fault zones crosscut many lithologies and operate over broad ranges of pressure, temperature, and strain rates. Rocks at high-pressure and high-temperature conditions flow viscously by a number of deformation mechanisms whereas at low-pressure and low-temperature conditions rocks crack, fracture, lose cohesion and slide frictionally. At intermediate crustal depth, deformation is hence achieved by a complex spatial and temporal interplay between “viscous” and “brittle” processes. The interaction between these end-member cases where viscous flow cannot accommodate all the imposed displacement and abundant pervasive fracturing occurs leads to “semi-brittle” deformation. This session welcomes contributions investigating the interplay between brittle and viscous deformation processes from rock physics experiments, numerical and analog models, theoretical considerations, as well as field observations.