This first part of a four-part series of papers presents a review of the literature on high solidity ratio axial flow hydrofoil impellers. Interest in the application of high-flow, low power number, hydrofoil impellers, such as the Prochem Maxflo T and the Lightnin A315, has developed over recent years and has largely been stimulated by reports of improved fermentation performance relative to the Rushton disc turbine. Initially, the review briefly examines the characteristics of the radial flow Rushton turbine, as traditionally used in fermenters, with particular reference to its weaknesses. It also discusses the use of pitched blade turbines (mixed flow impellers) for air dispersion since these show some characteristics similar to those of the axial flow hydrofoils. The consideration of these two impeller types provides the background to explain the advantages and potential problems associated with high solidity ratio, axial flow, downward pumping hydrofoils. These aspects are discussed in relation to existing literature which is still relatively scarce. Finally, the advantages of retrofitting are introduced. Parts 2-4 report on a recent, detailed, fluid dynamic study of the Prochem Maxflo T and Lightnin A315 hydrofoil impellers which extends our knowledge of these important impeller types.