Cold water immersion is the biggest killer of sports people undertaking their sport. For example, 80 % of those that die in triathlons do so during the swim. The impact of cold on performance can include: subtle decrements in muscle function and co-ordination; increased substrate utilisation; incapacitation, drowning and sudden cardiac death. Cold affects performance by the stimulation of cold receptors in the skin and by the direct impact of cooling on nerves, muscle and connective tissue as well as the heart and brain. On entering cold water, sudden cooling of cutaneous cold receptors evokes hyperventilation and tachycardia, components of the “cold shock” response, which is often a precursor to drowning. If the face is also submerged the coincidental activation of cold shock tachycardia and diving response bradycardia can result in “Autonomic Conflict” and, in susceptible individuals, can predispose to sudden cardiac death. In cold air, freezing and non-freezing cold injury represent potential threats, but seldom to life. Provided an individual is reasonably well clothed and can remain active, hypothermia should not occur. However, if poorly clothed and inactive due to injury or exhaustion, then hypothermia represents a significant threat. Hypothermia affects cellular metabolism, blood flow, fluid shifts and neural activation. In comparison with heat and altitude, adaptation to cold remains something of a mystery, in part because of the historic ability of individuals to avoid cold exposure and therefore the stimulus to adapt. One sporting group who are repeatedly exposed to cold are the rapidly growing number of open water swimmers, including many triathletes. In theory humans may show metabolic (raised basal metabolic rate), hypothermic (undefended fall in deep body temperature) or insulative (increased body insulation via increased subcutaneous fat or reduced peripheral blood flow) adaptation to cold. Different aboriginal groups have been reported to have all of these types of adaptation. The type of adaptation developed has been suggested to vary with i. the fitness and body morphology of those being adapted (high fitness/low body fat: metabolic adaptation; low fitness: insulative adaptation; Bittel, 1992) and ii. the duration of exposure to cold, with adaptation progressing with time from the metabolic to hypothermic to insulative form (Skreslet & Aarefjord, 1968). The adaptation observed may also vary depending on whether or not exercise is performed during the repeated cold exposures or during the assessment of adaptation (Golden & Tipton, 1998). Typically, outdoor cold water swimmers, both children and adults, tend to demonstrate a hypothermic adaptation to cold when at rest in cold water and an insulative adaptation when exercising in cold water (Bird et al. 2015). Repeated exposure to cold increases thermal comfort (Golden & Tipton, 1988); whilst beneficial to performance, this can also be hazardous by disassociating the thermal state of the body from the drive to behaviourally thermoregulate (the most powerful of the thermoregulatory responses); it means that cold adapted individuals are poor at assessing how cold they are. Responses vary in their lability: the initial response to cold water immersion can be significantly attenuated by a small number of short exposures. The site of this habituation appears to be central to the peripheral cold receptors and once established the habituation lasts, in part, for at least 14 months. Repeated showers are a less effective stimulus for adaptation than repeated whole body (head out) immersions, with the rate of change of skin temperature on immersion apparently being an important determinant of the magnitude and specificity of the adaptation developed. Interestingly, the ability to breath hold on immersion in cold water can be improved by psychological skills training without repeated exposure to cold water (Barwood et al. 2007). In contrast, the decrement seen in neuromuscular function with cooling does not tend to improve with repeated exposure. The metabolic (shivering) response to cooling is often attenuated with repeated cold exposure; this habituation requires falls in deep body temperature and is specific to the deep body temperatures experienced when becoming adapted, with a normal, unadapted metabolic response being evoked if individuals cool themselves more than they are used to (Tipton et al. 2013). In contrast, the habituation of the initial response to cold water immersion is not as specific, with habituation to warmer water temperatures (15 °C) providing some habituation to colder temperatures (10 °C). Although different in nature, habituation of both the initial and metabolic responses to cold water improve swimming performance in cold water. Finally, “cross adaptation” can occur between cold and altitude, with repeated short term cold water exposures improving the responses of individuals at altitude. Evidence suggests that the mechanism for cross adaptation is located in the autonomic nervous system; this area is worthy of further investigation (Lunt et al. 2010).