We discuss dielectric and light scattering susceptibility spectra ofsimple glass formers at temperatures above as well as below thecritical temperature of the mode coupling theory (MCT). Close toTgthe systems are characterized by the presence of a pronounced excess wing(type A glass formers). The data are analysed within a phenomenologicalapproach, on the one hand, and within MCT, on the other. Among other workwe present a complete interpolation of the dielectric data for glycerol(Lunkenheimer et al2000 Contemp. Phys. 41 15). The crossover temperatureTx,defined by the emergence of the excess wing upon cooling, is extracted from thephenomenological analysis and found to agree well with the critical temperatureTc,extracted from the MCT analysis at high temperatures. BelowTxthe evolution of the susceptibility is characterized by a universal appearanceof the excess wing. No difference is observed for the non-fragile systemwith respect to fragile glass formers provided that the wing parametersare studied as a function of the correlation time τα.Finally, a generalized scaling for the susceptibility minimum is proposed which is aphenomenological extension of that of MCT but now also includes the data belowTc.