In this chapter, we will first provide an overview of human thermoregulatory response for maintaining core body temperature in cold environments, with some recent updates especially in the role of human brown adipose tissue (BAT) as a thermoeffector. The focuses on the inter- and intra-individual variation in the BAT activity and multiorgan contributions to cold-induced thermogenesis including shivering and nonshivering thermogenesis (NST) would be an original part. Mechanisms of cold-induced vasoconstriction are briefly summarized including the role of arterio-venous anastomoses (AVA), activation of the adrenergic neural function, and inhibition of nitric oxide vasodilator pathway. In addition, cold-induced vasodilation (CIVD), the paradoxical vasodilation response during cold immersion of distal extremities, is summarized with some recent updates in the potential physiological mechanism. The most important suggestion based on the laboratory studies is that a repeated cold exposure does not enhance CIVD response. Furthermore, as a physiological reaction other than thermoregulation, initial cold shock response at the onset of cold water immersion is summarized including the potential mechanism and habituation of the response. Next, regarding cold adaptation, categorization of cold acclimatization/acclimation into habituation of shivering, metabolic and insulative adaptation are summarized with some recent updates especially in the metabolic acclimation with enhanced NST and BAT activity. At the latter part of this chapter, we summarize physical performance impairment in cold environments such as maximal muscle strength, anaerobic power, endurance of submaximal exercise, and manual dexterities, with introducing potential underlying physiological mechanisms including thermoregulatory, neuromuscular, cardiovascular, and metabolic systems. Then, we introduce our recent studies on the greater contribution of anaerobic metabolism during submaximal exercise in cold and new training method with hypothermic skeletal muscle. Training in cold water utilizes the cold-induced physiological strain as an extra physiological load such as greater glycolytic metabolism even at low to moderate mechanical intensity. Our recent intervention study revealed that the hypothermic skeletal muscle training improves anaerobic power, even when moderate workload was repeated during the training session. Finally, we will introduce studies on cognitive performance in cold environments.