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Brittleness is an important parameter controlling the mechanical behavior and failure characteristics of rocks under loading and unloading conditions, such as fracability, cutability, drillability and rockburst proneness. As such, it is of high practical value to correctly evaluate rock brittleness. However, the definition and measurement method of rock brittleness have been very diverse and not yet been stan-dardized. In this paper, the definitions of rock brittleness are firstly reviewed, and several representative definitions of rock brittleness are identified and briefly discussed. The development and role of rock brittleness in different fields of rock engineering are also studied. Eighty brittleness indices publicly available in rock mechanics literature are compiled, and the measurement method, applicability and limitations of some indices are discussed. The results show that (1) the large number of brittleness indices and brittleness definitions is attributed to the different foci on the rock behavior when it breaks;(2) indices developed in one field usually are not directly applicable to other fields; and (3) the term“brittleness”is sometimes misused, and many empirically-obtained brittleness indices, which lack theoretical basis, fail to truly reflect rock brittleness. On the basis of this review, three measurement methods are identified, i.e. (1) elastic deformation before fracture, (2) shape of post-peak stress-strain curves, and (3) methods based on fracture mechanics theory, which have the potential to be further refined and unified to become the standard measurement methods of rock brittleness. It is highly beneficial for the rock mechanics community to develop a robust definition of rock brittleness. This study will undoubtedly provide a comprehensive timely reference for selecting an appropriate brittleness index for their applications, and will also pave the way for the development of a standard definition and measurement method of rock brittleness in the long term.