Experimental measurements and simulation results for the evolution of plastic deformation and hardening in micropillars are compared. The stress–strain response of high-symmetry Cu single crystals is experimentally determined using in situ micropillar compression. Discrete dislocation simulations are conducted within a two-dimensional plane-strain framework with the dislocations modeled as line singularities in an isotropic elastic medium. Physics-based constitutive rules are employed for an adequate representation of hardening. The numerical parameters entering the simulations are directly identified from a subset of experimental data. The experimental measurements and simulation results for the flow stress at various strain levels and the hardening rates are in good quantitative agreement. Both flow strength and hardening rate are size-dependent and increase with decreasing pillar size. The size effect in hardening is mainly caused by the build-up of geometrically necessary dislocations. Their evolution is observed to be size-dependent and more localized for smaller sample volumes, which is also reflected in local crystal misorientations.