A discrete dislocation model was used to simulate residual stress fields close to deformation twins in stainless steels. Dislocation pairs were distributed along an initially elliptical twin boundary and an iterative scheme used to allow the dislocations to relax towards positions where the internal shear stresses were below a friction stress. The relaxed twin shape was close to elliptical but the flanks were flatter and the twin tips where the grain boundaries are situated were somewhat blunted compared to an ellipse. A dislocation-based boundary element model was then used to assess the interaction between the stress field from such twins and a crack. The stress field near the tip of the crack was characterized in terms of mode I and mode II stress intensity factors. The effects of twin width, length, orientation and distance from the crack tip on the strength of the interaction were studied. Wider, shorter twins were found to induce the largest stress intensity at the crack tip when close to the crack tip and aligned perpendicular to the crack plane. The influence of a pair of deformation twins does not significantly exceed the influence of a single deformation twin.