This article presents a state-of-the-art review focusing on the current understanding of the multiscale multiphase phenomena inside Bubble Column Reactors (BCRs). Although many reviews are available on BCRs, little attention has been devoted to summarizing multiscale multiphase phenomena, which are common fundamental issues encountered in their applications. These issues range from the microscale of single bubble dynamics to the mesoscale of bubble swarms and up to the macroscale of the reactor. Understanding these phenomena in all relevant scales can help the rational design, scale-up and optimization of BCRs. The microscale bubble dynamics including the bubble shape and motion, the relevant forces involved and the single bubble mass transfer, is summarized. At the mesoscale, the hydrodynamics of a bubble swarm is influenced by the bubble-bubble or bubble-liquid interactions and hence the overall transport properties of a bubble swarm are not linearly related to that of a single bubble. The bubble swarm effect and the bubble breakage and coalescence mechanisms are discussed in detail. In the end, the macroscale or reactor scale dynamics is strongly governed by the interplay between microscale and mesoscale phenomena, but more research focusing on mesoscale phenomena will be particularly needed for improving our understanding of BCRs. (C) 2019 Elsevier Ltd. All rights reserved.