Models of plant hydraulic resistance are useful because they provide mechanistically anchored predictions of plant gas exchange and survival in response to environmental stress and ontogeny. Flow resistance in soil and plant xylem can be modeled relatively easily because the processes are largely physical and linked to environment and plant size in ways that can be readily characterized. Three hydraulic models are discussed. A soil-plant-atmosphere-continuum model predicts the sensitivity of plant gas exchange and survival in response to water stress. An allometric model improves on previous "metabolic scaling theory" and predicts water use and productivity as a function of plant size and functional type. In development is a model linking vascular structure to its vulnerability to cavitation. The potential exists to combine all three in a comprehensive bottom-up representation of species-specific responses to environment and ontogeny.