This chapter presents fundamental knowledge on human wear trials for assessing clothing comfort performance. The chapter begins with a brief introduction to firefighting protective clothing systems and heat transfer mechanisms. Experimental protocols and criteria for the appropriate selection of human subjects (test persons) for human trial studies are discussed. Various parameters used to characterise human thermophysiological responses (e.g., skin temperature, core temperature, mean body temperature, heat storage, heart rate, oxygen consumption, metabolic heat, sweating) are addressed and their measurement methodologies are explicitly discussed. Perpetual responses (e.g., thermal sensation, comfort sensation, skin wetness sensation, perceived exertion) and associated rating scales, as well as heat strain indices (e.g., physiological strain index, psychological strain index), are also presented. Finally, human trial case studies for analysing protective clothing and effects of physical activity, environmental conditions, and different protective clothing on human thermophysiological and perceptual responses are explored and summarised.