Fillers and additives can influence the crystallization behavior of polymers. The additives can be classified into inactive nucleating agents, that have no significant influence on the crystallization kinetics, and active nucleating agents, that accelerate the crystallization process. The active nucleating agents can be particulate additives like carbon nanotubes. Other types of nucleating agents or additives form self-assembly structures that may accelerate crystallization. For example, with polypropylene (PP), the influence of the nucleating activity of sorbitol-type nucleating agents, carbon-nanotubes, calcium carbonate and nano-diamonds is studied by isothermal crystallization experiments in the temperature range between 0 °C and 130 °C using fast differential scanning calorimetry (FDSC). An acceleration factor is defined which quantifies the influence of the nucleation agent on the overall crystallization rate. It is shown, all additives investigated influence the crystallization process of PP. The activity of the additives strongly depends on the temperature. The fillers are only active nucleating agents in a dedicated temperature range.