This book focuses on iron (Fe) in cells, since it is the most abundant metal in living system and is involved in a variety of physiological events. Both excess and shortage of iron cause serious diseases such as anaemia, cancer and neuronal degeneration. The cellular systems consisting of specific proteins strictly control the iron contents through the iron dynamics in cells including absorption, sensing, storage, transport and usage. Resulting from a 5-year project on Integrated Biometal Science in Japan, this book not only documents the latest research but also fills a gap between chemical understanding and our real life.