Heat stress and body water deficits (hypohydration) can impair both submaximal and maximal intensity aerobic exercise performance (Sawka et al., 2011). This paper examines the roles of hot skin (>35°C) alone and hypohydration (>2% body mass) on impairing submaximal aerobic performance. For this paper, we define hot skin as 35oC and above, warm skin as 30 to 34.9°C and cool / cold skin as <30°C. We recognize that skin temperature effects are a continuum and the Tsk to Tc gradient alters these relationships. Warm-hot skin is associated with high skin blood flow requirements and hypohydration is associated with reduced cardiac filling, both of which act to reduce aerobic reserve. In euhydrated subjects, hot skin alone (with a modest core temperature elevation) impairs submaximal aerobic performance (Ely et.al. 2009). Conversely, aerobic performance is sustained with core temperatures >40°C if skin temperatures are cool-warm when euhydrated (Ely et.al. 2010). No study has demonstrated that high core temperature (~40°C) alone, without coexisting hot skin, will impair aerobic performance (Cheuvront et.al. 2010). In hypohydrated subjects, aerobic performance begins to be impaired when skin temperatures exceed 27°C, and even warmer skin exacerbates the aerobic performance impairment (-1.5% for each 1°C Tsk) (Kenefick et.al. 2010). Hot skin and hypohydration may act through a variety of mechanisms including increased thermal discomfort, osmoreceptor / baroreceptor stimulation, reduced aerobic reserve, elevated perception of effort and altered afferent signal processing. We conclude that: 1) hot skin (high skin blood flow requirements from narrow Tsk to Tc gradients), is the “primary” factor impairing submaximal aerobic performance when euhydrated; 2) hypohydration impairs submaximal aerobic performance when skin temperatures ~27°C, and even warmer skin exacerbates (-1.5% for each 1°C Tsk) these decrements; and 3) high core temperature (~40°C) alone does not impair aerobic performance.