|ABSTRACTSuspensions of solid particles in viscous liquids can vary from low-viscosity liquids to wet granular materials or soft solids depending on the solids loading. When the particles are very concentrated, these mixtures are called “dense suspensions”.Dense suspensions often exhibit shear thickening, an increase in apparent viscosity as the shear rate is increased. In its most extreme form, abrupt order of magnitude increases in viscosity over such a narrow range in shear rate occur, leading to the term discontinuous shear thickening (DST).We focus on simulations of a recently developed “lubricated-to-frictional” rheology in which the interplay of viscous lubrication, repulsive surface forces, and contact with frictional interactions between particle surfaces provides a scenario to explain DST. The large change in properties is associated with the breakdown of lubricating films between particles, with activation of Coulomb friction between particles. Comparison of the simulations with experiments and a theoretical model (Wyart & Cates, Phys. Rev. Lett. 2014) based on the activation of friction between particles show good agreement, and all point toward DST being a nonequilibrium phase transition; a brief overview of our present understanding of the microstructural basis for this behavior in terms of contact force correlations will be provided.