Particle agglomeration in fluidized bed systems has been observed in several industrial processes. Bed particles tend to agglomerate when the cohesive force between the particles is strong enough to compare with the other acting forces (gravitational, drag). This cohesive force may arise because of different types of interactions: van der Waals, electrostatic, capillary, viscous, sintering, adhesive, chemical reaction, and so on. The most extensively reported case in the literature is concerned with the fluidized bed combustion of biomass, waste or low rank coals, containing significant amounts of low-melting compounds (typically alkali metals) in the ash. The occurrence of bed agglomeration in such systems implies the unscheduled shut down of the reactor and costly maintenance operations. Therefore, a great deal of research has been devoted to understand and characterize agglomeration during fluidized bed combustion. This short review is focused on this specific system and tries to summarize the present status of understanding of the mechanisms leading to agglomeration, as well as the influence of the different operating variables on this phenomenon. In addition, because of their great practical importance, the possible early detection techniques and operational countermeasures are also briefly described.