The importance of thermal comfort of a helmet related to heavy activities is considered to be the next to impact protection. Extensive research has been undertaken on the latter by introducing new materials/structures and computer simulations, including development of test standards. However, the helmet thermal comfort research is relatively limited. Therefore, there is a need to develop comprehensive testing methodology and numerical modelling to study the thermal comfort quantitatively. In this research programme, a novel test rig was developed by using microsensor technology to measure multi-point temperature and relative humidity (rH) inside a cricket helmet. The experimental tests were carried out on the limited ventilated and well ventilated cricket helmets at moderate and high ambient conditions in order to obtain the in-helmet micro climate thermal and moisture mappings. Results were presented in curve chart and contour plot formats, in which variations of the in-helmet temperature/rH and hot/wet spots could be clearly observed. Human subjective warmth and moisture sensations and comfort perception were also recorded and linked to the digital measurements of the in-helmet temperature and rH. Through such linkage, the equivalent sensation and perception indexes were established as tools to evaluate the predicted temperatures and rHs from the finite element (FE) parametric studies. Digital laser scanner was used to help the creation of 3D models of the head, helmet structure and air pocket and air channels' between the head and the helmet. The heat transfer and mass diffusion FE analyses showed reasonably good correlations of the in-helmet temperatures and rHs with the experimental results. Using validated computer models, parametric studies were carried out to cover various ventilation opening sizes, shapes and locations. The predicted in-helmet temperatures and rHs related to various ventilation opening configurations were processed to draw out the equivalent warmth/moisture sensation and comfort perception index scales. Such predicted scales were compared with the scales obtained from experimental measurements to judge whether the opening configuration is favourable to thermal comfort. The helmets with favourable opening configuration were subsequently subjected to impact modelling to evaluate their impact resistance. Finally, a helmet with optimised thermal comfort and necessary impact protection was recommended. This approach would assist the designing process of new types of helmet prior to prototype making or production. It is believed that the above approach will save a lot of man power and time and thus shorten the new product development cycle. The experimental methodology, finite element modelling and parametric study approach developed in this research programme can be used to study other types of helmet.