The evolution of salt tolerance is interesting for several reasons. First, since salt-tolerant plants (halophytes)
employ several different mechanisms to deal with salt, the evolution of salt tolerance represents a fascinating case study in the
evolution of a complex trait. Second, the diversity of mechanisms employed by halophytes, based on processescommonto all
plants, sheds light on the way that a plant’s physiology can become adapted to deal with extreme conditions. Third, as the
amount of salt-affected land increases around the globe, understanding the origins of the diversity of halophytes should
provide a basis for the use of novel species in bioremediation and conservation. In this reviewwepose the question,howmany
times has salt tolerance evolved since the emergence of the land plants some 450–470 million years ago? We summarise the
physiological mechanisms underlying salt-tolerance and provide an overview of the number and diversity of salt-tolerant
terrestrial angiosperms (defined as plants that survive to complete their life cycle in at least 200mM salt). We consider the
evolution of halophytes using information from fossils and phylogenies. Finally, we discuss the potential for halophytes to
contribute to agriculture and land management and ask why, when there are naturally occurring halophytes, it is proving
to be difficult to breed salt-tolerant crops