The adverse impacts of invading alien organisms are widely recognized as one of the major threats to biodiversity and are receiving growing recognition as a major socioeconomic threat. The hydrological impacts of alien plants have received less attention, despite growing evidence of their significance. The wide range in plant growth forms and physiology among invading species suggests that estimation of the hydrological impacts could be difficult. The concept of limits to evaporation was developed to help organize our general scientific understanding of the hydrological implications of changes in vegetation. It provides a way of reducing this complexity to the factors most likely to be the major determinants of evaporation from vegetation in a given situation. It distinguishes between physical factors (1) availability of energy from solar radiation and advection, (2) availability of soil moisture at landscape and habitat scales, and (3) raindrop size, and biological factors (4) plant size, including height and rooting depth, and (5) plant physiology, including seasonality of the leaves and drought tolerance. Studies of the hydrological impacts of vegetation change invasion show that changes in vegetation structure and seasonality can have significant impacts on water resources at both habitat and landscape scales. Invasions can also have significant impacts where the invaded habitat has more water available within the rooting depth than adjacent areas.