Charles V (Spanish: Carlos; German: Karl; Italian: Carlo; Latin: Carolus; Dutch: Karel) was ruler of both the Spanish Empire as Charles I from 1516 and the Holy Roman Empire as Charles V from 1519, as well as of the lands of the former Duchy of Burgundy from 1506. He voluntarily stepped down from these and other positions by a series of abdications between 1554 and 1556. Through inheritance, he brought together under his rule extensive territories in western, central, and southern Europe, and the Spanish viceroyalties in the Americas and Asia. As a result, his domains spanned nearly 4 million square kilometres and were the first to be described as "the empire on which the sun never sets".
Charles was the heir of three of Europe's leading dynasties: the Houses of Valois-Burgundy (through his paternal grandmother), Habsburg, and Trastámara (his maternal grandparents were the Catholic Monarchs of Spain). He inherited the Netherlands and the Free County of Burgundy as heir of the House of Valois-Burgundy. As a Habsburg, he inherited Austria and other lands in central Europe. He was also elected to succeed his grandfather, Maximilian I, as Holy Roman Emperor, a title held by the Habsburgs since 1440. From the Spanish House of Trastámara, he inherited the Crown of Castile, which was developing a nascent empire in the Americas and Asia, and the Crown of Aragon, which included a Mediterranean empire extending to Southern Italy. Charles was the first king to rule Castile and Aragon simultaneously in his own right, and as a result he is often referred to as the first king of Spain. The personal union under Charles of the Holy Roman Empire with the Spanish Empire was the closest Europe would come to a universal monarchy since the death of Louis the Pious (778–840).
Charles's Spanish dominions were the chief source of his power and wealth, and they became increasingly important as his reign progressed. In the Americas, Charles sanctioned the conquest by Castillian conquistadores of the Aztec and Inca empires. Castillian control was extended across much of South and Central America. The resulting vast expansion of territory and the flows of South American silver to Castile had profound long term effects on Spain.
Charles was only 56 when he abdicated, but after 40 years of energetic rule he was physically exhausted and sought the peace of a monastery, where he died at the age of 58. Upon Charles's abdications, the Holy Roman Empire was inherited by his younger brother Ferdinand, who had already been given the Austrian lands in 1521. The Spanish Empire, including the possessions in the Netherlands and Italy, was inherited by Charles's son Philip II of Spain. The two empires would remain allies until the 18th century (when the Spanish branch of the House of Habsburg became extinct).