king is a head of state, who may or may not, depending on the style of government of a nation, exercise monarchal powers over a nation, usually called a kingdom or a realm. A king is the second highest sovereign title, only looking up to an emperor. The female equivalent of king is queen; although the term "queen" may refer to one ruling as a monarch in her own right, a queen regnant, or to the wife of a king, a queen consort. A queen who becomes the reigning monarch because the king has died, has become debilitated, or is a minor, is known as a queen regent. The husband of a queen regnant is sometimes styled the king consort but is more commonly styled the prince consort. A king or queen may wear a crown or carry other regalia (symbols of office). Historically the term "king" has been used for the rulers of very small peoples and territories.Such a king (Sumerian lugal, Semitic sharrum, Latin rex, Greek basileus, Sanskrit raja, Germanic kuningaz) could be a tribal leader or chief, or the tyrant of a city state. Often, the king will not only have a political function, but the same time a religious one, acting as high priest or divine king. Tribal leaders continue to be referred to as king also into the modern period, e.g. Maquinna, king of perhaps 2000 Nootka people in the early 20th century.

A queen regnant (plural "queens regnant") is a qualifying reference to a female monarch (queen) possessing and exercising all of the monarchical powers of a ruler, in contrast to a "queen consort", who is the wife of a male reigning as monarch and who is without any official powers of state. In Ancient Egypt, Pacific cultures, and even in historical European countries, as noted below, women monarchs have been given the title king or its equivalent, such as pharaoh, when gender is irrelevant to the office. Also the Byzantine Empress Irene called herself basileus (βασιλεύς), 'emperor', rather than basilissa (βασίλισσα), 'empress'. And Jadwiga of Poland was crowned as Rex Poloniae, King of Poland. Among the Davidic Monarchs of Judea, there is mentioned a single queen regnant, Athaliah, though the Bible regards her negatively as an usurper. The much later Hasmonean Queen Salome Alexandra (Shlomzion) was highly popular. Technically a male king also may be a king regnant or a king consort—but this distinction is unusual and, for example, has been used only twice in the history of the British monarchy and its predecessor monarchies. In all current monarchies that allow a queen to take the throne, the husband of such a queen is not titled king, generally ranking as a prince. The husband of Queen Mary I of England and the first two husbands of Queen Mary I of Scotland were all created kings consort of their wives' realms. The husband of Mary II, Queen of England and Ireland, and Queen of Scots, was named king regnant co-sovereign with her, as William III and II. The latter arrangement was the only occasion of co-sovereignty in Britain. Accession of a regnant occurs as a nation's order of succession permits.

Methods of succession to queendoms, kingdoms, tribal chiefships, and such include nomination when the sitting monarch or a council names an heir, primogeniture when the children of a monarch or chief become regents in order of birth from eldest to youngest, and ultimogeniture when the children become regents in the reverse order of birth from youngest to eldest. The scope of succession may be matrilineal, patrilineal, or both; or, rarely, open to general election when necessary. The right of succession may be open to men and women, or limited to men only or women only. The most typical succession in European monarchies from the Late Middle Ages through most of the twentieth century was male-preference primogeniture; i.e., the order of succession cycled through the sons of the monarch in order of their birth, followed by the daughters or grandsons. Historically, many realms forbade succession by women or through a female line in obedience to the Salic law, and some still do. No queen regnant ever ruled France, for example. Only one woman, Maria Theresa ruled the Holy Roman Empire, she held the title Holy Roman Empress first by marriage and was the de facto ruler for forty years. As noted in the list below of widely-known ruling queens, many ruled in European monarchies. In the waning days of the twentieth century, Sweden, Norway, Belgium, and the Netherlands amended their acts of succession to primogeniture with no preference as to sex. In some cases the change does not take effect during the lifetimes of people already in the line of succession at the time the law was passed. In China, Wu Zetian became the Chinese Empress Regnant and established the Zhou Dynasty (also known as Wu Zhou 武周) after dismissing her sons and becoming the Empress Regnant. Although the Chrysanthemum Throne of Japan is barred to women, this has not always been the case; eight of the ruling empresses of Japan are listed below.