There are a number of design considerations which influence the selection of a suitable pitch and trough shape for a new spiral separator. The separation requirements impose certain constraints on the total design envelope and mutual interaction between some of the requirements and the conflict between metallurgy and materials handling requirements often defines a narrow range of permissible geometries. Capacity requirements determine the diameter of a spiral trough but fundamental fluid dynamic considerations play a major role in the selection of pitch and trough shape and the paper considers some of the issues in detail. Where the dynamics of the separation lead to excessive accumulation of solids at high slurry densities, various types of flow deflectors have been adopted either to restore fluidity to the material or to assist in securing a better quality of product. Apart from the trough, the design of suitable ancillaries such as feed boxes, auxiliary splitters and product collection facilities can exert a significant influence on both the operation of the spiral and the quality of separation achieved. These features are discussed in the paper and illustrated with examples.