The thermal insulation properties of clothing systems can be defined through physical measurements using thermal manikins or through wear trials using human test subjects. One objective of the European Subzero project was to define the relationship between physically measured thermal insulation values of cold-protective clothing and the corresponding physiological reactions on human test subjects. Four cold-protective clothing ensembles, intended for use in temperatures between 0 and −50°C, were measured with manikins in eight European laboratories and on human test subjects in four of these laboratories. The results showed that reasonably good reproducible values from the manikin tests can be achieved (CV <8%); however, the fit of the clothing on the manikin is a critical factor. There were greater individual differences in the wear trial results. Comparing the results from the manikin and the wear trials, good agreement in the thermal insulation values was shown if the amount of accumulated sweat was low. In these situations, which are normal when using cold protective clothing, the thermal comfort can also be determined with good accuracy by means of mathematical models based on manikin results. Special situations, e.g. for highly perspiring wearers, strong wind, or high friction between garment layers, need specific modelling; some suggestions have been made as a result of the Subzero project, but further research is required.