Energy generated at Avista's Monroe Street and Upper Falls power plants is transmitted via large underground cables to the Post Street Substation, where it is distributed throughout the company's electrical system. Serving as the control center for Monroe Street and Upper Falls, this building houses personnel who oversee both plants.

Constructed in 1909, the Post Street Substation has also served as a warehouse and sheltered streetcars. Powered by electricity from Avista's hydroelectric facilities, streetcars were a popular form of Spokane-area transportation from the late 1800s until they were removed from service in 1936.

Designed by the famed architect Kirtland Cutter, this Romanesque brick structure with large, recessed arch windows is an excellent example of Spokane's early industrial architecture.

The four ornate iron domes, or cupolas, which once graced the corners of the substation roof, were donated to the U.S. government during the World War II scrap metal shortage. Post Street stands today as a prominent part of Avista's "living history."