Iron is an essential nutritional element for all life forms. Iron plays critical roles in electron transport and cellular respiration, cell proliferation and differentiation, and regulation of gene expression. Two emerging new functions for iron are its necessary role in supporting transcription of certain key genes required for cell growth and function [eg, nitric oxide synthase, protein kinase C-&bgr;, p21 (CIP1/WAF1)] and its complex role in hematopoietic cell differentiation. However, iron is also potentially deleterious. Reactive oxygen species generated by Fenton chemistry may contribute to major pathological processes such as cancer, atherosclerosis, and neurodegenerative diseases. Iron-generated reactive oxygen species may also function in normal intracellular signaling. Therefore, roles of iron are both essential and extraordinarily diverse. This symposium explores this diversity by covering topics of iron absorption and transport, the regulation of gene expression by iron responsive proteins, the cellular biology of heme, hereditary hemochromatosis, and clinical use of serum transferrin receptor measurements.