The mechanisms of plasticity in metal nanowires with diameters below 100 nm are reviewed. At these length scales, plasticity in face-centered-cubic metals subjected to uniaxial loading is dominated by dislocation nucleation from free surfaces, which has been studied extensively by molecular dynamics. These simulations show that nanowires can deform in a variety of ways including slip via perfect dislocations, partial dislocations and deformation twins. The competition between these mechanisms can be explained primarily through the Schmid factor and material properties, although surface orientation and roughness also contribute. The strength of these materials is very high and can be described by classical nucleation theory which predicts strong temperature and geometry dependence as well as a weak strain rate dependence. Additionally, nanowires exhibit, through twinning or phase transformation, pseudo-elastic and shape-memory behaviors which are attributed to their small size and the surface stress. The plasticity of nanowires subject to torsion and bending as well as those composed of body-centered-cubic metals are also summarized.