Current theory and experimental data are reviewed on size effects in the plastic deformation of fully dense and cellular metals. After summarising observed size effects in torsion, bending and indentation of solid metals, the underlying microstructural reasons for an observed size effect and the resulting strain gradient theories of plasticity are reviewed. The role of geometrically necessary dislocations is emphasised and the need for higher order boundary conditions is demonstrated. For the case of metallic foams, size effects are apparent in indentation tests, in shear and compression tests on sandwich panels and in tensile tests on notched specimens. The underlying reason is the bending of cell walls. Comparisons are made with finite element simulations of random microstructures and with a dilatant strain gradient theory of plasticity. Again, higher order boundary conditions are needed to model the size effect in a satisfactory manner.