This article critically reviews progress in the understanding of strain hardening during the deformation stage II of pure face-centered cubic crystals since its discovery in the mid-1950s. A wealth of models attempted explaining why stage II exhibits a linear slope on the base of specific dislocation configurations and interactions associated with short-or long-range internal stresses. Slip trace observations and more recent investigations on dislocation avalanches led to the identification of intermittent elementary slip events that are now investigated in terms of dislocation mechanisms. Numerical estimates showed that the overcoming of junctions and locks accounts for most of the flow stress and the hardening rate, as assumed by the present forest models. A multiscale analysis of stage II is outlined with emphasis on the modeling of dislo-cation mean free paths by dislocation dynamics simulations.