Evaporation is the largest single component of the terrestrial hydrological cycle and its accurate estimation is important in water resource management, crop yield forecasting and understanding the links between land use changes and climate. Most practical models of evaporation first calculate potential evaporation and then try to correct this to actual evaporation using a range of factors. This paper demonstrates the problems inherent in this approach and concludes that better progress will be made by using surface resistance to calculate actual evaporation directly. Conventional evaporation formulae apply to land surfaces which are covered with dense uniform vegetation, yet globally this is not very common. This paper also presents a brief review of more recent evaporation models which are able to predict evaporation from sparsely vegetated and/or heterogeneous terrain, more representative of the majority of the world's vegetation.