The Bond operating work index is in common use for assessment of comminution efficiency. This method was developed for rod and grinding ball mill circuits and is not well suited to AG/SAG mills or the wide range of comminution devices now in common use. Traditional measures of efficiency only consider the net motor power of the drive unit. However, ancillary processes such as materials handling and classification can consume a substantial fraction of motor power. Wear resistant liners and grinding media are also consumed by the comminution process. These consumables themselves require substantial energy to mine, refine and to manufacture. This embedded (or indirect) energy can be of similar order to motor power. This paper outlines several ways to estimate of comminution efficiency. These methods can be applied to a wide range of equipment. The paper also describes how to estimate indirect energy consumption. (C) 2009 Elsevier Ltd. All rights reserved.