Drought tolerance is a nebulous term that becomes more nebulous the more closely we look at it, much as a newspaper photograph does when viewed through a magnifying glass. From the vantage point of an ecologist the features that distinguish xerophytic from mesophytic vegetation are clear. We can all tell that a cactus is more drought tolerant than a carnation. But when we look at crop plants, the features that confer drought tolerance are far from clear. The main reason for the contrast is that the traits we associate with xerophytes typically concern survival during drought, whereas with crops we are concerned with production — and insofar as the term “drought tolerance” has any useful meaning in an agricultural context, it must be defined in terms of yield in relation to a limiting water supply.