With its 40.000 inhabitants, Desio is one of the most important municipalities in the province of Monza and Brianza as well as the nerve center of the texile industrial district of the area. Although the discovery of a sacrificial altar dating back to the pre-Roman period reveals that the area was inhabited by Ligurian-Celtic populations, the place name has Roman origins and derives from the Latin phrase "ad decimum", referring to the settlements located along the imperial road connecting Milan to Como.
During the Middle Ages it was part of the county of Martesana, mentioned for the first time in 931; in 1277 it also framed an important battle between the Visconti family and the Torriani one for the control of Milan. In the first half of the XIV century Bernabò Visconti built a castle in Desio. In 1385 Gian Galeazzo annexed the Pieve church of Desio to the jurisdiction of the mayor of Milan. In 1511 the city was sacked by the Landsknecht, immortalized in a drawing by Leonardo da Vinci and preserved in the British royal collection. In 1613 it became a marquis under Philip III of Spain. Moreover, Desio is the birthplace of Achille Ratti, Pope Pius XI; his native house is today a museum and research center.
- the Basilica of the Saints Siro and Materno, founded by the bishop Giovanni Bono in 649. The present building dates from the XVII-XVIII century, projected by the architects Pellegrini, Ciniselli, Merlo and Galliori; it was also enlarged in the apse, at the end of the last century. Inside it is possible to admire XIX-XX century paintings by Mauro Conconi and Giuseppe Riva, the main altar (1744) by Nava and Antignani and some XVI century tapestries. The Gothic style bell tower dates back to the XV century and is equipped with a well-known concert of bells, realized in 1843.
- the XVIII century Church of Santa Maria, with valuable XVI-XVII century paintings and a baptismal font made from an ancient holy water font from the Church of San Francesco;
- the Sanctuary of the Holy Cross, in neo-Gothic style, designed by Spirito Maria Chiappetta and opened in 1913. It is located in the same place where once was the Oratory of San Bartolomeo, coeval with the foundation of the basilica;
- the small Church of Sant'Apollinare (XVII century), in the hamlet of San Giorgio;
- the XVII century Oratory of San Giuseppe, in the homonymous village, annexed to the residence of the family Ferrario-Brambilla-Buttafava. It is in Baroque style.
- the external Chapel of the cemetery, of ancient origin, although the present building dates back to the first years of the XIX century;
- Villa Cusani Tittoni Traversi, valuable example of Neoclassical style architecture. Together with the Royal Villa in Monza, it represents a fantastic example of country patrician villa. The current building is the result of two different renovations that transformed the original Baroque building: the first one by Giuseppe Piermarini, who gave to the villa a Neoclassical shape; the second one by the architect Pelagio Pelagi who worked at the garden. It currently houses the municipal offices and the public library, while the garden is public;
- the native house of Achille Ratti, Pope Pius XI, houses a museum of memorabilia, including a radio donated by Guglielmo Marconi.