Because of the balance of forces in fluidised beds, particle interactions can have a strong effect on their microscopic and macroscopic behaviour, leading to agglomeration and defluidisation. Three types of particle interactions are reviewed: van der Waals forces, liquid bridge forces and sintering. Sintering is qualitatively different in its effects because it is a time-dependent process. The observed effects of these three types of interactions on fluidisation behaviour are described and explained in terms of simple models.