The combination of attractive mechanical properties and high corrosion resistance make metastable austenitic stainless steels useful in various applications. They have rather low yield strength in solution-treated condition, but strain-harden significantly due to both conventional dislocation strengthening and a partial phase transformation to martensite, during cold deformation. The deformation-induced martensitic transformation (DIMT) and the exceptional strain-hardening, hence, invokes the so-called Transformation Induced Plasticity (TRIP) effect that prevent localized neck formation and give excellent ductility and formability, the strain-hardening further provide high strength after cold forming. Clearly, the significant effect of DIMT on mechanical properties suggests that a profound understanding of DIMT through experiments and physical-based materials modeling is vital to fully utilize the merits of the metastable austenitic stainless steels in technical applications. This chapter provides an introduction to DIMT in metastable austenitic stainless steels and, moreover, it aims at providing some perspectives on current activities in the field. In particular, from an experimental viewpoint, the methodologies to investigate DIMT as well as the microstructure and its mechanical response are discussed; from a modeling perspective, first-principles and thermodynamic calculations of the stacking-fault energy, and structural modeling using the phase-field method is elaborated on.