Drilling is one of the main activities of upstream sector in oil and gas industry, where the well is drilled and hydrocarbon is extracted to the surface. This activity involves many technical disciplines, which are strictly connected together from well planning until the hydrocarbon production. It is critical to have a thorough understanding of the mechanical response of the rock that will be drilled in order to avoid drilling instability problems that can result in additional cost. The analysis of wellbore stability starts with the characterization of the rock. The interpretation of triaxial tests in the framework of selected strength criteria, gives the rock properties that rule the mechanical response during drilling operations.