Iron is the second most abundant metal in the earth’s crust and among the most commonly used metals in industry. Iron is of fundamental importance in electrochemistry due to its rich redox chemistry, which has seen applications in bioelectrochemistry as well as materials in fuel cell electrocatalysts, batteries, and capacitors. This chapter briefly outlines the basic physical and chemical properties of iron including its fundamental thermodynamics and its redox behavior in solution. The electrochemistry of iron is briefly discussed with special emphasis on power sources, including nickel–iron, and iron–air batteries, FeS2 cathodes, iron-nitrogen electrocatalysts for fuel cells and microbial fuel cells based on iron redox proteins.