The dynamics of a gas-solid fluidized bed are described. A model of a shallow bed suggested in previous work is critically examined and shown to have serious limitations in the light of experimental evidence. A lumped parameter model is derived in the paper and theoretical results derived from this model are shown to compare favourably with experimental results, particularly with regard to natural frequency of oscillation. Formation of bubbles in the bed are also examined and it is confirmed that bubble formation is cyclic and is related to the natural frequency of the bed. The experimental procedures based on cross-correlation using a pseudo-random binary sequence and direct measurement of bubbles using a capacity probe are described.