The TRIP – effect in metastable austenitic steels is caused by a moderate local martensitic transformation, connected with an extraordinary increase e.g. of the uniform elongation in tensile testing. In this case the martensite formation causes an additional hardening effect, preventing the local deformation as well as damaging during plastic deformation. The main conditions of a marked TRIP – effect is a low flow stress in the undeformed state, a high strengthening exponent, a continuous transformation up to about 20 % martensite and a high resistance against damaging (e.g. crack formation and propagation). The martensite transformation starts in glide or shear bands and their crossings also at temperatures clearly above the conventional Md – temperatures, reducing the stress local concentrations and so preventing damaging. Furthermore, high martensite contents can be detected along the fracture path, indicating the hindering of the crack propagation by the transformation. To demonstrate the influence of damaging on the TRIP – effect the deviations from the so-called Considere-criterion for uniform elongation can be used. Related to the deformation the damaging under TRIP – conditions is minimal.