- Much of the recent effort in developing phenomenological constitutive descriptions of size dependent plasticity for crystalline solids has associated size dependence with geometrically necessary dislocations. However, there are a variety of perspectives on geometrically necessary dislocations and the characterization of their role in size dependent plastic flow phenomena is controversial. Therefore, a Viewpoint Set on this issue, including materials and mechanics points of view, and experimental and theoretical perspectives was deemed timely. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.