The biogeochemical cycle of iron in the ocean has been extensively studied over the past three decades, with significant advancements in understanding the sources, sinks, and cycling processes of iron. Iron is a critical nutrient for marine productivity, and its availability influences carbon sequestration and atmospheric CO2 levels. The ocean's iron cycle involves multiple sources, including dust, coastal and shallow sediments, sea ice, and hydrothermal fluids. Iron is rapidly recycled in the upper ocean by various organisms, with up to 50% of the soluble iron pool being turned over weekly in some regions. Bacteria play a crucial role in dissolving particulate iron and releasing iron-binding ligands that complex with iron, maintaining its solubility. Sinking particles also scavenge iron from the water column. The balance between supply and removal processes determines dissolved iron concentrations, which exhibit nutrient-like profiles. Iron profiles in different ocean basins show constancy in deep waters, but inter-basin variability exists, influenced by sampling methods and analytical techniques. The role of colloids and iron-binding ligands in iron distribution has been elucidated, with distinct classes of ligands found in different parts of the water column. Iron supply mechanisms include atmospheric dust deposition, coastal and riverine inputs, and hydrothermal activity. The biological recycling of iron, known as the "ferrous wheel," involves bacteria, grazers, and viruses, rapidly mobilizing the biogenic iron pool. Particulate iron undergoes remineralization as it sinks, replenishing the deep-water inventory. Climate change may alter the iron cycle by affecting ocean circulation and iron supply, but the specific impacts remain uncertain. Advances in iron stable isotopic analysis and global surveys of trace elements will help better understand these changes.The biogeochemical cycle of iron in the ocean has been extensively studied over the past three decades, with significant advancements in understanding the sources, sinks, and cycling processes of iron. Iron is a critical nutrient for marine productivity, and its availability influences carbon sequestration and atmospheric CO2 levels. The ocean's iron cycle involves multiple sources, including dust, coastal and shallow sediments, sea ice, and hydrothermal fluids. Iron is rapidly recycled in the upper ocean by various organisms, with up to 50% of the soluble iron pool being turned over weekly in some regions. Bacteria play a crucial role in dissolving particulate iron and releasing iron-binding ligands that complex with iron, maintaining its solubility. Sinking particles also scavenge iron from the water column. The balance between supply and removal processes determines dissolved iron concentrations, which exhibit nutrient-like profiles. Iron profiles in different ocean basins show constancy in deep waters, but inter-basin variability exists, influenced by sampling methods and analytical techniques. The role of colloids and iron-binding ligands in iron distribution has been elucidated, with distinct classes of ligands found in different parts of the water column. Iron supply mechanisms include atmospheric dust deposition, coastal and riverine inputs, and hydrothermal activity. The biological recycling of iron, known as the "ferrous wheel," involves bacteria, grazers, and viruses, rapidly mobilizing the biogenic iron pool. Particulate iron undergoes remineralization as it sinks, replenishing the deep-water inventory. Climate change may alter the iron cycle by affecting ocean circulation and iron supply, but the specific impacts remain uncertain. Advances in iron stable isotopic analysis and global surveys of trace elements will help better understand these changes.