We first review some of the definitions of brittleness index (BI) that have been used in the recent literature concerning oil and gas exploration and production from low porosity, low permeability rocks. We will then argue that the definitions characterizing the BI of rocks either by their elastic properties, by their mineralogical composition or by their strength characteristics, are all equivalent and typically result in a higher BI assigned to quartz-rich rocks than to clay-rich lithologies. Therefore the majorities of recent definitions of BI are simply a rock- type indicator and are useful as such. However, the separation of rocks into brittle/ductile lithologies on the basis of a calculated BI is not necessarily an indicator of brittle or ductile failure during hydraulic stimulation. We therefore propose that brittleness index is potentially an unfortunate choice of words and can, at worst, be misleading. We will then show how incorporation of elastic and strength properties into a geomechanical model, which additionally includes the stress state and pore pressure, can be used to determine (i) whether hydraulic fractures are likely to be contained in the resource layer and or will grow out of zone (ii) whether rock fails predominantly in tension or in shear during hydraulic stimulation, and (iii) whether hydraulic stimulation will predominantly create new fractures or is likely to re-activate pre-existing fractures and other planes of weakness (such as bedding boundaries).