Brittleness is often used to guide selection of fracturing targets in unconventional reservoirs. There are however several definitions of brittleness parameters, with varying degree of scientific basis. Using parameters which lack scientific basis to solve operational problems that require fundamental physical understanding is a risky engineering practice. It is therefore important to identify to what extent and in case how brittleness can be used in practical field operations, and how it may be quantified in a reliable way. Rock mechanical laboratory experiments have been performed with different shales, including Mancos shale (as an analogue to gas shales), Pierre Shale (representative of soft overburden shales) and various field shale cores from the North Sea. Unconfined and triaxial compression plus Brazilian tensile tests have been done, and brittleness indices have been studied by use of different techniques to quantify the elastic fraction of the measured strain prior to failure, and by measuring the stress drop after passing peak stress. The experiments show that brittleness in anisotropic shale depends on the direction of loading with respect to the symmetry plane, and on the character of the failure process itself: Axial splitting and shear failure along the bedding plane in unconfined and triaxial tests as well as clean tensile failures in Brazilian tests give high brittleness values. The selected brittleness parameters decrease with increasing confining pressure, as expected from basic rock mechanics. The rate of decrease in brittleness with confining stress appears correlated with increased shale porosity, or with lower shale strength. In fully saturated shales, brittleness is higher in drained than undrained conditions and it is shown that this difference can be linked to plasticity occurring during undrained pore pressure evolution. Plasticity plays an important role to stabilize a borehole against collapse. Borehole stability modeling shows that a decrease in brittleness (corresponding to increased plasticity) may stabilize a borehole which otherwise could not have been drilled in stable conditions. In fracturing operations, the local state of stress, geological heterogeneities (including natural fractures) and the anisotropic poro-elasto-plastic behaviour of the shale formation, controls the resulting efficiency. This includes brittleness, but requires a better understanding of the complex mechanics of the fracturing process.