Evidence for the clustering of alkalis in mixed alkali silicate glasses from recent Small Angle X-ray Scattering, X-ray Absorption Fine Structure and Molecular Dynamics experiments is discussed. As a result of clustering different types of alkali will associate in the glass structure leading to different configurations which in turn will have a disordering effect on nearest neighbour oxygens as well as an influence on the hopping probability. Both the local structure of the different alkalis present in the glass and their respective diffusivities are observed to change with mixed alkali composition providing a structure-function description of the Mixed Alkali Effect.