This discussion follows the flow of iron from dietary intake, through absorption, to organ uptake, to cellular use, to release, and to iron re-uptake or loss. Known and putative diseases will be discussed at each step. Although discussed separately, these processes both occur simultaneously and influence each other. Like many other fields of cellular and molecular science, the challenge is both to reduce a complex problem to parts that can be studied and then to reassemble the parts into a clinically useful whole.