Abstract Transfer of water vapor, carbon dioxide, and other trace gases between plants, animals, and the atmosphere takes place by molecular and turbulent diffusion. This chapter uses principles from fluid dynamics to develop methods for relating mass and heat transfer, and applies the methods to analyze mass exchange between leaves and the atmosphere. Mass exchange between the air in greenhouses and similar structures and the atmosphere is also analyzed. Mass diffusion through stomatal pores on leaves is a process limiting photosynthesis, respiration, transpiration, and the uptake of pollutant gases. The concept of stomatal resistance is introduced and measured resistances are compared with theoretical predictions. Water vapor transfer through animal skin, coats, human clothing, and through pores in artificial “breathable” fabric is also analyzed by resistance analogs.