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The transpiration coefficient (K-c) method estimates evapotranspiration as a proportion of the evapotranspiration of a reference crop growing in ideal conditions. This approach was designed for irrigated crops and assumes that plants are not subjected to resource limitations. Other assumptions are that plants have high leaf area index and little stomatal resistance to water loss. These conditions are not common for arid-land vegetation. However, mainly due to its simplicity, some studies have proposed the use of transpiration coefficients as a method of determining evapotranspiration in arid environments. In this article, the documented applications of the K-c method in arid environments and their accuracy are reviewed. We also critically discuss the physiological and agronomic concepts that support the K-c method as they relate to water-limited environments. The K-c method typically overestimates water use when plants encounter suboptimal conditions of soil water because it does not consider stomatal regulation and plant adaptations to drought. We conclude that, although the transpiration coefficient method is simple to implement and widely recognized, it is not suitable for determining evapotranspiration of vegetation adapted to arid conditions.