To treat tensile instabilities the main points of the theory of uniqueness and bifurcation in elastic-plastic solids are outlined. Also simple theories for localization of plastic flow are discussed. Instability predictions are highly sensitive to the material model applied. Ductile fracture in metals involves the nucleation and growth of micro-voids to coalescense. Such porous ductile materials also show localization of plastic flow. Analyses for low stress triaxiality show failure mechanisms involving collapse of voids into micro-cracks that subsequently interact.