Holy Roman Emperor Ferdinand II and Danish King Christian IV sign the Treaty of Lübeck ending Danish intervention in the Thirty Years' War.

The Holy Roman Emperor, originally and officially the Emperor of the Romans during the Middle Ages, and also known as the Roman-German Emperor since the early modern period, was the ruler and head of state of the Holy Roman Empire. The title was held in conjunction with the title of King of Italy from the 8th to the 16th century, and, almost without interruption, with the title of King of Germany throughout the 12th to 18th centuries.

Ferdinand II was Holy Roman Emperor, King of Bohemia, Hungary, and Croatia from 1619 until his death in 1637. He was the son of Archduke Charles II of Inner Austria and Maria of Bavaria, who were devout Catholics. In 1590, when Ferdinand was 11 years old, they sent him to study at the Jesuits' college in Ingolstadt because they wanted to isolate him from the Lutheran nobles. A few months later, his father died, and he inherited Inner Austria–Styria, Carinthia, Carniola and smaller provinces. His cousin, Rudolf II, Holy Roman Emperor, who was the head of the Habsburg family, appointed regents to administer these lands.

This is a list of monarchs of Denmark, that is, the kings and queen regnants of Denmark. This includes:

• The Kingdom of Denmark
  • Personal union of Denmark and Norway (1380–1397)
• The Kalmar Union (1397–1536)
  • Union of Denmark, Norway and Sweden (1397–1523)
  • Union of Denmark and Norway (1523–1536/1537)
• The United Kingdoms of Denmark–Norway (1536/1537–1814)
• The Kingdom of Denmark (1814–present)
  • Iceland
  • Greenland
  • Faroe Islands

Christian IV was King of Denmark and Norway and Duke of Holstein and Schleswig from 1588 until his death in 1648. His reign of 59 years and 330 days is the longest in Scandinavian history.