A review is presented on the role of dislocation cores and planar faults in activating the nonbasal deformation modes, <c+a> pyramidal slip and deformation twinning, in hcp metals and alloys and in D0₁₉ intermetallic compounds. Material-specific mechanical behavior arises from a competition between altemate defect structures that determine the deformation modes. We emphasize the importance of accurate atomistic modeling of these defects, going beyond simple interatomic energy models. Recent results from both experiments and theory are summarized by discussing specific examples of Ti and Mg single crystals; Ti-, Zr-, and Mg-base alloys; and Ti₃Al ordered alloys. Remaining key issues and directions for future research are also discussed.