This paper is concerned with the coating of solids in fluidised bed equipment. The different physical mechanisms taking place in the process are incorporated into simple theoretical models which can be used to explain the observed phenomena. They are of significant importance for the design of equipment and for the quality of the coating. Conclusions are drawn for the optimisation of the design and the operating parameters. The role of solids movement in the coating process is illustrated by experimental results and realised examples of equipment. This paper also points out several unexplained questions concerning coating of solids in fluidised beds. New laboratory methods are required which, together with theoretical models, allow for the estimation of the fundamental process and design parameters.