Iron is the most common element in the earth as a whole, and the fourth most common in the Earth's crust. It is produced as a result of stellar fusion in high-mass stars, and it is the heaviest stable element produced by stellar fusion because the fusion of iron is the last nuclear fusion reaction that is exothermic. Iron is the most widely used metal, and iron compounds, which include ferrous and ferric compounds, have several uses as well.

Iron has been used since ancient times, though not as early as bronze or the other copper related alloys. Iron is ubiquitous in modern life; it is used primarily for its structural strength. Pure iron is soft (softer than aluminium), but the material is significantly strengthened by addition of minute amounts of impurities, such as carbon. Alloying iron with appropriate small amounts (up to a few per cent) of other metals and carbon produces steel, which can be 1,000 times harder than pure iron. Iron is smelted in a blast furnace, where ore is reduced by coke to metallic iron.

Elemental iron is reactive; it oxidizes in air to give iron oxides, also known as rust. The rusting of iron and iron alloys is undesirable, and has a major economic impact. Unlike many other metals which form passivating oxide layers, iron oxides occupy more volume than iron itself. Thus, iron oxides flake off and expose fresh surfaces for corrosion. Iron oxide mixed with aluminium powder can be ignited to create a thermite reaction, used in welding and purifying ores.

Iron exists from oxidation state −2 to + 6, although +2 and +3 are the most common. It forms binary compounds with the halogens and the chalcogens. Among its organometallic compounds, ferrocene was the first sandwich compound discovered. Iron plays an important role in biology, forming complexes with dioxygen as hemoglobin and myoglobin; these two compounds are common oxygen transport proteins in vertebrates.

Mechanical properties of iron and its alloys are evaluated using a variety of tests, such as the Brinell test, Rockwell test, or tensile strength tests, among others; the results on iron are so consistent that iron is often used to calibrate measurements or to relate the results of one test to another. Those measurements reveal that mechanical properties of iron crucially depend on purity: Purest research-purpose single crystals of iron are softer than aluminium. Addition of only 10 parts per million of carbon doubles their strength. The hardness increases rapidly with carbon content up to 0.2% and saturates at ~0.6%. The purest industrially produced iron (about 99.99% purity) has a hardness of 20–30 Brinell.

Iron represents perhaps the best-known example of allotropy in a metal. There are three allotropic forms of iron, known as α, γ and δ.

As molten iron cools down it crystallizes at 1538 °C into its δ allotrope, which has a body-centered cubic (bcc) crystal structure. As it cools further its crystal structure changes to face-centered cubic (fcc) at 1394 °C, when it is known as γ-iron, or austenite. At 912 °C the crystal structure again becomes bcc as α-iron, or ferrite, is formed, and at 770 °C (the Curie point, Tc) iron becomes magnetic. As the iron passes through the Curie temperature there is no change in crystalline structure, but there is a change in "domain structure", where each domain contains iron atoms with a particular electronic spin. In unmagnetized iron, all the electronic spins of the atoms within one domain are in the same direction; the neighboring domains point in various directions and thus cancel out. In magnetized iron, the electronic spins of all the domains are aligned, so that the magnetic effects of neighboring domains reinforce each other. Although each domain contains billions of atoms, they are very small, about 10 microns across.

Iron is of greatest importance when mixed with certain other metals and with carbon to form steels. There are many types of steels, all with different properties, and an understanding of the properties of the allotropes of iron is key to the manufacture of good quality steels.

Alpha iron, also known as ferrite, is the most stable form of iron at normal temperatures. It is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C).

Above 912 °C and up to 1400 °C α-iron undergoes a phase transition from bcc to the fcc configuration of γ-iron, also called austenite. This is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146 °C). This form of iron is used in the type of stainless steel used for making cutlery, and hospital and food-service equipment.