This paper introduces a strain gradient theory of plasticity, which is based on the concept of statistically stored and geometrically necessary dislocations. The theory fits within the framework of couple stress theory and involves a single material length scale \( l \). Minimum principles are developed for both deformation and flow theory versions of the theory, which reduce to conventional \( J_2 \) deformation and \( J_3 \) flow theory in the limit of vanishing \( l \). The theory is used to calculate the size effect associated with macroscopic strengthening due to a dilute concentration of bonded rigid particles and to predict the effect of void size on macroscopic softening due to a dilute concentration of voids. Constitutive potentials are derived to describe these effects. The paper also reviews couple stress theory, outlines the deformation and flow theory versions of the strain gradient theory, and discusses boundary layers near interfaces or rigid boundaries. The theory predicts that strain gradient effects have a minor influence on void softening and growth but can lead to significant strengthening effects for rigid particles.This paper introduces a strain gradient theory of plasticity, which is based on the concept of statistically stored and geometrically necessary dislocations. The theory fits within the framework of couple stress theory and involves a single material length scale \( l \). Minimum principles are developed for both deformation and flow theory versions of the theory, which reduce to conventional \( J_2 \) deformation and \( J_3 \) flow theory in the limit of vanishing \( l \). The theory is used to calculate the size effect associated with macroscopic strengthening due to a dilute concentration of bonded rigid particles and to predict the effect of void size on macroscopic softening due to a dilute concentration of voids. Constitutive potentials are derived to describe these effects. The paper also reviews couple stress theory, outlines the deformation and flow theory versions of the strain gradient theory, and discusses boundary layers near interfaces or rigid boundaries. The theory predicts that strain gradient effects have a minor influence on void softening and growth but can lead to significant strengthening effects for rigid particles.