Book excerpt: Iron is a major constituent of the earth crust. However, under alkaline conditions commonly found in arid and semi-arid environments iron becomes unavailable to plants. When plants are affected by a shortage of iron their leaves become yellow (chlorotic), and both plant growth and crop yield are reduced. The roots of plants affected by iron deficiency may develop a series of responses directed to improve iron uptake, such as increased proton excretion and iron reduction capabilities or excretion of iron chela tors called siderophores. Iron deficiency affects major crops worldwide, including some of major economic importance such as fruit trees and others. Correction of iron deficiency is usually implemented through costly application of synthetic chelates. Since these correction methods are very expensive, the competitivity of farmers is often reduced and iron deficiency may become a limiting factor for the maintenance, introduction or expansion of some crops. In spite of the many years devoted to the study of iron deficiency, the knowledge of iron deficiency in soils and plants is still fragmentary in many aspects. We have only incomplete information on the processes at the molecular level that make some plant species and cultivars unable to take and utilize iron from the soil, whereas other plants grow satisfactorily under the same conditions.