This lecture will mainly address the issues associated with mixing at the large scale in bioreactors using large scale free suspension culture as an example. It will particularly consider problems that become steadily more important as the scale increases to > 10000 L at cell densities approaching 107/ ml. However, very few papers have been written that are actually based on large scale studies and the few that do rarely address any of the issues quantitatively. Hence, it is necessary very often to extrapolate from small scale work and this lecture will try to pull the two types of studies together. ‘Shear sensitivity' due to agitation and bursting bubbles appears no longer to be considered as such a major issue by industry. Homogeneity is recognised as becoming increasingly important with respect to top surface addition of chemicals for pH control and of nutrient feed for fed-batch operation at the largest scale. The spatial and temporal variations that result with top feeding can be greatly reduced by sub-surface feeding but this mode of feeding gives rise to ‘cleaning in place' and related issues. There are still major problems with cell retention/recycle systems at these scales, either because of fouling, capacity or potential and different ‘shear sensitivity' issues. Fed-batch operation gives rise to cell densities that have led to the use of oxygen and enriched air to meet oxygen demands. This strategy, in turn, gives rise to a CO2 evolution rate that impacts on pH control, pCO2 and osmolality issues. These three closely related issues are difficult to resolve but if higher sparge rates and agitation intensities could be used to achieve the necessary oxygen transfer, they would largely disappear. Thus, ‘shear sensitivity' at higher air flow rates and agitation intensity is still a perception, the validity of which will also be closely considered in the light of the extensive small scale studies that have been carried out. The points that differentiate the mixing issues in animal cell bioreactors from those in bacterial and mycelial fermentations will also be very briefly mentioned.