Criteria for grain boundary dislocation nucleation are developed. A bicrystal containing two grain boundaries is placed under varying triaxial stress states using molecular dynamics. The local resolved shear, normal, and co-slip stresses needed for grain boundary dislocation nucleation are found. A framework is developed to detect the slip system grain boundary dislocation nucleation occurs on. A survey of the different ways grain boundary dislocation nucleation occurs in the sample shows a single grain boundary can nucleate dislocations in a rich variety of ways. Using the nucleation system and resolved stress values, criteria for grain boundary dislocation nucleation on different slip systems are developed. The proposed form of nucleation criterion suggests the activation stress has a linear dependence one the resolved shear, normal, and co-slip stresses. A residual analysis largely validates the efficacy of the proposed linear model. We show that the nucleation slip system cannot be predicted by a maximum Schmid factor analysis due to the non-Schmid resolved normal and co-slip terms. We show that a system's global pressure generally fails to predict nucleation; a local stress in the grain being nucleated into should be used. Using the nucleation criteria for each slip system, a yield surface for dislocation nucleation is built for the grain boundary used in this work.