Eeonomie plants range from those that are eommercially utilised by the industrial nations of the world to those that safeguard the survival of hunter-gatherer aboriginal eommunities. In a world of inereasing human and livestock populations, land and environment degradation, species loss and climatic change, it is becoming inerea- singly important to work for the sustainable management of the world's natural resourees. This is especially important for the arid and semi-arid regions where any abuse of the harsh environment is unforgiving. Plant resourees are especially important sinee they proteet the environment, provide habitats for both flora and fauna, and direetly or indireetly provide the resourees for man's survival. An understanding of how plants are adapted to survive is an essential tool for the better management of the environment. Sinee there are over 20000 useful plants in the arid and sem i-arid regions of the world, of which only a relative few have been even partially investigated, the writer makes no apologies for whieh plants have been included or ignored. All the plants mentioned in the introductory ehap- ters marked with an asterisk are diseussed more fully in Chapter 11.