Iodine is a trace element responsible for the correct development of the cow’s organism, milk production, and reproduction. This microelement ensures the normal function of the thyroid gland, and is indispensable for the synthesis of the thyroid hormones triiodothyronine and thyroxine, which are responsible for healthy metabolism, especially energy metabolism. Iodine exhibits strong disinfecting and bactericidal properties and is a powerful antioxidant. In a feed ration, it may be present as non-organic and organic compounds that are later almost fully absorbed in the rumen and omasum. Non-organic forms are much better absorbed than organic ones. Living organisms do not synthesise iodine, so the main causes of its deficiency are insufficient amounts of iodine in soil and water. Iodine deficiency in ruminants may also be caused by feeding plants with goitrogens and by the excess of calcium in the feed ration. The maximum iodine concentration allowed in feed rations is 5 mg/kg DM. Iodine deficiency in dairy cattle can lead to hypothyroidism, manifested by the birth of stillborn or weak calves with an overgrowth of the thyroid gland (goiter), reproductive disorders, and an increased incidence of mastitis and hoof diseases. Iodine intoxication in cattle usually occurs as a result of administering too much of a mineral additive containing iodine, using a few different forms of supplements with this element or an uneven mixing of mineral additives with other feed ingredients. The aim of this review is to review the literature and to provide a comprehensive overview of the knowledge concerning the role iodine in cattle.