Neat and binary molecular glass formers are investigated by means of broadband dielectric spectroscopy with the aim of gaining a systematic understanding of the molecular slowing down, which is characteristic for the glass transition. By combining frequency and time domain techniques a dynamic range from 10-6, Hz up to 109, Hz is covered. Particular attention is drawn to the fact that different types of secondary relaxations may appear during the process of supercooling. In certain simple glass formers such secondary processes are clearly of intermolecular origin, and it turns out that in mixtures of small and large molecules secondary relaxations can be systematically altered by changing the concentration of the constituents. Thus, the relation between different types of secondary processes may be clarified. It is typical of glass forming substances that the decay of orientational correlations is non-exponential, and, correspondingly, the dielectric loss spectra are broadened as compared to a simple Debye-like relaxation process. The latter effect is particularly pronounced in binary glass formers, and one aim of the present work is to provide and apply an appropriate tool for a line shape analysis of the relaxation spectra in both neat end binary systems. On the other hand, the nature of spectral broadening is investigated by means of non-resonant dielectric hole burning, which allows to distinguish between heterogeneous and homogeneous dynamics in a system. It turns out that inbinary glass formers dynamic heterogeneities are particularly pronounced and that the effects of dielectric hole burning in both neat and binary systems are very well described within the framework of a model of selective local heating.