The plastic deformation of metals results in strain hardening, that is, an increase in the stress with increasing strain. Materials engineers can provide a simple approximate description of such deformation and hardening behavior. In his perspective, [Gumbsch][1] discusses work by [ Madec et al .][2] who have undertaken the formidable task of computing the physical basis for the development of strain hardening by individually following the fate of all the dislocations involved. Their simulations show that the collinear dislocation interaction makes a substantial contribution to strain hardening. It is likely that such simulations will play an important role in guiding the development of future engineering descriptions of deformation and hardening. [1]: http://www.sciencemag.org/cgi/content/full/301/5641/1857 [2]: http://www.sciencemag.org/cgi/content/short/301/5641/1879