The microgravity and hypobaric environments encountered in space flight will alter the convective and evaporative heat and mass transfer coefficients that influence thermal comfort. In this paper, models of heat and mass transfer between the human body and the environment are presented. Three mode of convection are identified: (1) free convection governed by the temperature gradient between the skin and the environment and by the acceleration of gravity, (2) forced convection governed by the ambient gas movements, and (3) mixed convection combining the previous two modes. Mixed convection is shown to prevail in most situations on Earth. In microgravity the contribution of free convection to the convective heat exchange is negligible and only forced convection must be considered. The ‘classic’ Lewis analogy between the convective heat and mass transfer is commonly used on Earth to calculate the mass transfer coefficient from the convection coefficient. The Lewis relationship is shown to be valid whatever the mode of convection. The effects of microgravity and low ambient pressure upon water vapor diffusion are analyzed and a special case study is performed by considering the Skylab environment which combines microgravity and hypobaric environments.