A review of granular flow in rotating drums, with a specific focus on the underlying rheology, is presented. The rich coexistence of flow regimes in tumbling mills - the industrial application of rotating drums - highlight the difficulty in obtaining key flow field measurements like velocity and volume concentration distributions, with non-invasive techniques proving the most useful. The mixture of experimentally derived scaling laws underscore the difficulty in defining a suitable granular rheology for tumbling mills. The visco-plastic rheology proposed by Jop et al. (2006) denotes a major step forward in the understanding of dense granular rheology with the scalar form having some experimental corroboration in rotating drums. Unfortunately, the success is militated by the mixed results in subsequent numerical and experimental studies. More specifically, it fails the full tensorial test with notable lack of prediction of the well-known hysteresis between flow initiation and cessation, and the expected phase transition to cataracting flows. Beyond the zeroth order approximation of the visco-plastic rheology, we also explore the pragmatic approaches of depth averaged modelling that include kinetic theory based ingredients to successfully capture the two phase transitions. (C) 2016 Elsevier Ltd. All rights reserved.