The Duchy of Milan was a constituent state of the Holy Roman Empire, in what is now northern Italy. Created in 1395, when it included twenty-six towns and a wide rural area lying between the hills of Montferrat and the Venetian Lagoon, the Duchy was conquered by the Austrians during the 18th century War of the Spanish Succession, and in 1714 it was ceded to Austria by the Treaty of Baden. It remained an Austrian possession until 1796, when it was conquered by a French army under Napoleon Bonaparte, and ceased to exist a year later as the result of the Treaty of Campo Formio, when Austria ceded it to the new Cisalpine Republic.

After the defeat of Napoleon, the Congress of Vienna of 1815 restored many other states which he had destroyed, but the Duchy of Milan was not among them. Instead, its former territory became part of the Kingdom of Lombardy–Venetia, with the Emperor of Austria as its king. In 1866 what was left of this kingdom was annexed by the new Kingdom of Italy.

The Duchy of Milan (Ducatus Mediolani), was created 1 May 1395, when Gian Galeazzo Visconti, Lord of Milan, purchased a diploma for 100,000 Florins from King Wenceslaus. It was this diploma that installed, Gian Galeazzo as Duke of Milan and Count of Pavia. At its foundation the duchy included 26 towns and spanned from the hills of Montferrat to the Lagoons of Venice, and included all the former towns of the Lombard League.
During the rule of the Visconti and Sforza, the duchy had to defend its territory against the Swiss, the French and the Venetians, until the Betrayal of Novara in 1500 when the duchy passed to the French-claim of Louis XII.

Milan (Italian: Milano [miˈlaːno] ( listen); Lombard: [miˈlan]; Latin: Mediolanum) is the second-largest city in Italy and the capital of Lombardy as well as of the province of Milan. The city proper has a population of about 1.3 million, while its urban area is the 5th largest in EU and the largest in Italy with an estimated population of approximately 5 million. The growth of many suburbs and satellite settlements around the city proper, following the Italian economic miracle of 1950s–60s and massive commuting flows, suggest that socioeconomic linkages have expanded well beyond the boundaries of its administrative limits and its agglomeration, creating a region of approximately 9,3 million people. It has been suggested that the Milan metropolitan area is part of the so-called Blue Banana, the area of Europe with the highest population and industrial density. Its health care and education system is considered one of the best of Europe.