Al-base marginally glass-forming systems such as Al92Sm8 and Al88Fe5Y7 that have been vitrified by rapid quenching often crystallize via the initial formation of large number densities of Al-nanocrystals at temperatures that are below the respective glass transition. Calorimetric investigations and quantitative TEM analyses concerning the so-called nanocrystallization reaction indicate that the primary transformation results from diffusion-limited growth of quenched-in nuclei. Quite surprisingly, some easy glass-forming alloy systems that permit the synthesis of vitreous bulk samples by melt-quenching, such as Mg65Cu25Y10, also form comparably large number densities of nanocrystals during the initial stages of the devitrification process. However, as opposed to the marginally glass-forming Al-base alloys, the nanocrystal formation in bulk glass-forming alloys occurs in the undercooled melt, i.e. at temperatures that are above the respective glass transition temperatures. Here, we compare the nanocrystal formation process and its kinetics for different groups of alloy systems in order to analyze the mechanism and the major determining parameters that govern the phase formation sequence and the nanoscaled microstructure evolution in light-metal base glass-forming alloys.