An effort has been made in this book to estimate the time for fatigue of a human operator via a quantitative study based on the workload. Workload in turn depends on the condition of the operator and environmental conditions, which is reflected in the muscle temperature. Simple yet comprehensive mathematical models for heat transfer in human body were developed for both steady as well as transient states.The models were also validated against the results available in the literature. Parametric studies involving the condition of the operator and the environment were also carried out. From the mathematical model, muscle temperature required for the estimation of the time for fatigue of the operator was determined. Three case studies from a multinational company were considered which involve different types of task. The time for fatigue has been estimated for the three cases by mathematical calculation and compared with the actual time for fatigue of the operator. This book in meant for students and researchers in the field of human factors engineering and human body heat transfer.