Discrete dislocation calculations suggest that statistical effects are important in the modeling of crystal plasticity. These include: (i) effects of source limited plasticity; (ii) statistical variations of dislocation sources and obstacles in small volumes; and (iii) the sensitivity of the discrete dislocation predictions to small perturbations. Implications of these for phenomenological constitutive models and for statistical theories of the collective behavior of dislocations are discussed. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.