Siderophores (from the Greek: “iron carriers”) are defined as relatively low molecular weight, ferric ion specific chelating agents elaborated by bacteria and fungi growing under low iron stress. The role of these compounds is to scavenge iron from the environment and to make the mineral, which is almost always essential, available to the microbial cell. Research in this field began about five decades ago, and interest in it has accrued with the realization that most aerobic and facultative anaero- bic microorganisms synthesize at least one siderophore. Sid- erophores have been related to virulence mechanisms in micro- organisms pathogenic to both animals and plants. In addition, they have clinical applications and are possibly important in agriculture. For an introduction to the field, the reader is referred to one of the comprehensive monographs on the subject (1, 2).