Until the late 19th century, Oh bej! Oh bej! (the most important and traditional fair of Milan) was held in Piazza Mercanti.
The square houses four main buildings:
- the "Broletto Nuovo", also known as Palazzo della Ragione, occupies the north-eastern side. It was built in 1233, and served as the "broletto", i.e., the administrative headquarters of the city. In the original layout of the piazza, which extended further to the north-east, the palace was located in the centre.
- the Gothic Casa Panigarola, also known as "Palazzo dei Notai" (Notary's Palace), built in the 15th century, is on the south-western side;
- the Baroque Palazzo delle Scuole Palatine, built in the 17th century and designed by Carlo Buzzi, is on the south-eastern side; it replaced a former building known as "Scuole del Broletto" ("Broletto Schools");
- also on the south-eastern side is the Loggia degli Osii, built in 1316 for Matteo I Visconti and designed by Scoto da San Gimignano; this was also an administrative seat, and included the parlera, i.e., the balcony from which the authorities addressed the population.
The 16th century Palazzo dei Giureconsulti, now located in Via Mercanti, used to mark the north-eastern side of the piazza before it was redesigned. It was built in 1561 on a design by Vincenzo Seregni; the tower of the building is much older, dating back to the 13th century (although it was largely restored in the 17th century).
At the centre of the square is a 16th century pit, surmounted by two 18th century columns. The pit was originally adjacent to the Palazzo dei Giureconsulti; where it stands now, a large stone was found, known as the "pietra dei falliti" ("bankrupts stone"), where those guilty of bankrupt would have their naked bottom exposed as a penance