Methods for the characterization of mould slag crystallization with special emphasis on the single/double hot thermocouple technique (SHTT/DHTT) are reviewed. In the continuous casting process of steels, horizontal heat transfer is mainly influenced by the crystallization behaviour of the mould flux film. Here, both precipitation of crystals out of a liquid phase and devitrification of the glassy film in contact with the mould are of main interest. Therefore, various investigation methods are implemented to characterize different slag properties related to crystallization: a viscometer for determining the break temperature, differential thermal analysis (DTA), confocal scanning laser microscopy, and the water-cooled copper finger test. For near-service conditions, DHTT reveals the most detailed information, including not only the crystallization or devitrification temperature but also the morphology as well as the crystallization velocity. Due to improvements in the device and the representation of the results, a comparison of different samples is possible. Nevertheless, the application field of SHTT/DHTT is restricted to slag systems with low contents of evaporating components. Furthermore, the time required for data analysis is significantly longer than that required for other methods, e.g. DTA. Therefore, the application of DHTT is mainly advisable for mould slag research and development, whereas DTA can also be used for incoming inspections.