Church of the Assumption of the Virgin Mary and of St. Joseph, commonly known as the Carmelite Church.
It was built in 1692-1701 to the plan of Jozef Szymon Bellotti, erected over the site of an older (1643) wooden church.
The Neoclassical facade dates from about 1782. Inside - 18th century high alter designed by the famous architect of Dutch descent, Tylman van Gameren.
This church was the site of Fryderyk Chopin's first employment. He was invited to give a recital on the wonderful organ inside.
During World War II the church escaped severe damage and was only slightly damaged. As one of the very few architectural monuments in Warsaw it remained original.
After the war it served as a procathedral until the reconstruction of St. John's Cathedral in the Old Town.

This photo is for the September Scavenger Hunt challenge:
Sept. 16. calendar or church
More entries are in the forum http://forum.pbase.com/viewtopic.php?f=16&t=38699&sid=55a7ef30516b0c5783f6fe9cb9213a2b