The fraction and microstructure of spontaneous and deformation-induced martensite in three austenitic stainless steels with different austenite stability have been investigated. Samples were quenched in brine followed by cooling in liquid nitrogen or plastically deformed by uniaxial tensile testing at different initial temperatures. In-situ ferritescope measurements of the martensite fraction was conducted during tensile testing and complemented with ex-situ X-ray diffractometry. The microstructures of quenched and deformed samples were examined using light optical microscopy and electron backscattered diffraction. It was found that annealing twins in austenite are effective nucleation sites for spontaneous alpha'-martensite, while deformation-induced alpha'-martensite mainly formed within parallel shear-bands. The alpha'-martensite formed has an orientation relationship near the Kurdjumov-Sachs (K-S) relation with the parent austenite phase even at high plastic strains, and adjacent alpha'-martensite variants were mainly twin related (< 111 > 60 degrees or Sigma 3).