Stronger and more ductile steels are increasingly demanded in important industry sectors such as automotive, gas transport ad power generation. One of the classes of Advanced High Strength Steels (AHSS) that are attracting researchers and industries are the so-called Quenching and Partitioning (Q&P) steels. The microstructure of these steels, formed by laths of low-carbon martensite separated by films of carbon enriched retained austenite, are responsible for these properties. Although both constituents are certainly, steel developers and scientists do not get an agreement about which one of these constituents is actually controlling the response of the material upon deformation. The way in which retained austenite is believed to contribute to the strain (work) hardening of these steels is via the TRansformation Induced Plasticity (TRIP) effect, by which austenite transforms to martensite during deformation. Therefore, the actual research question is: Is the TRIP-effect playing a role in the response of Q&P steels upon deformation? The crystal structure of the retained austenite is different from the martensite, and these differences can be detected with X-ray diffraction. In this project, the evolution of the crystal structure of different Q&P specimens were studied in Situ using a X-ray diffraction meter equipped with a micro-tensile tester and if necessary a sample heater. The formation of martensite during the tensile tests was divined form the decrease of austenite diffraction peaks. These experiments allowed detecting the possible occurrence (or absence) of the TRIP-effect.