The properties of dense plant cell suspensions, such as high viscosity, shear sensitivity, cell aggregation, and tendency to foam, present significant technical challenges for bioreactor design and operation. As in all fermentation processes, reactors used for large-scale plant cell culture must provide adequate mixing and mass transfer at nondamaging levels of hydrodynamic shear. In stirred vessels, this means that the impeller must be operated with restricted power input and speed, while still achieving complete gas dispersion and solids suspension. Although many experimental studies of conventional and novel impellers have been carried out, no clear guidelines for impeller choice have emerged. In this paper, an engineering analysis is used to identify which agitators and operating regimes have greatest potential for high-density plant culture applications, irrespective of whether they have been tested experimentally. Ways of improving impeller performance by modifying internal reactor geometry are also discussed.