Large eddy simulations (LES) of the flow in stirred tanks were performed. One of the advantages with LES is that it can provide details of the flow field that cannot be obtained with so-called Reynolds averaged equations and the corresponding models. Simulations were done on both single and multi-impeller systems. Both Rushton and curved-blade radial impellers were studied in both a fixed and a rotating frame of reference. The results show that the periodicity is much stronger and present in a larger part of the vessel for the Rushton turbine than for a curved-blade turbine.