Key factors associated with the production of biomass pellets are pellet durability and operational cost of the pellet mill. This paper reviews parameters of the pellet die configuration and operational variables that influence the performance of the pellet mill and the quality of the produced pellets. A die is specified by design parameters such as surface area and distribution pattern of press channels in the die, and correspondingly the press channel parameters are defined by length/diameter ratio and inlet angle and depth. In this paper, physical mechanisms are reviewed with respect to each of the design parameters, as well as the operational conditions of the pellet mill. The review of the literature has highlighted gaps in several areas that need further research. Specifically, the design of the conical press channel inlet and the surface area which appear to be key parameters in optimizing the process. (C) 2019 Elsevier B.V. All rights reserved.