Disturbances of iron metabolism, particularly iron deficiency, are among the most commonly overlooked or misinterpreted diseases. This is due to the fact that the determination of transport iron in serum or plasma, which used to be the test in conventional diagnosis, does not allow a representative estimate of the body's total iron reserves. A proper estimate was formerly possible only by the costly and invasive determination of storage iron in the bone marrow, however, sensitive, well-standardized immuno-chemical methods for the precise determination of the iron storage protein ferritin in plasma are now available. Since the secretion of this protein correctly reflects the iron stores in the majority of cases, these methods permit fast and reliable diagnosis, particularly of iron deficiency conditions. This book is intended to contribute to a better understanding of the main pathophysiological foundations and diagnostic principles.