The ocean contains about 50 times the amount of carbon in the atmosphere, and currently takes up about a third of the anthropogenic carbon emissions. Primary production in the ocean is responsible for maintaining a gradient in carbon content between the surface ocean and the deep sea that determines atmopheric pCO2 over timescales of thousands of years. But what determines the strength of the marine primary production? Over the last decades it has become clear that the micro-nutrient iron is a key determinant of biological productivity in large areas of the worlds ocean. Unlike the macronutrients phosphorus and nitrogen, iron is very particle-reactive and quickly lost from the ocean; dust deposition to the ocean surface is the main process that brings in new iron. However, both the loss of iron and its input into the ocean are strongly influenced by the complicated chemistry of iron in seawater. In the talk I will try to give an overview what we currently know and what we do not understand yet about the cycling of iron in the ocean, and its role in present and possibly future marine productivity.