St. Louis is an independent city in the U.S. state of Missouri, located at the confluence of two of the world's great rivers, the Mississippi River and the Missouri River. One of the world's great river cities, St. Louis has a diverse multi-cultural population, and has architecture, festivals, sports, historical sites, and other aspects which draw visitors from around the world. St. Louis is the second largest city in the state of Missouri, but has the largest metropolitan area. Sometimes written as Saint Louis, the city is named for King Louis IX of France. St. Louis is famous for its multiple French and German influences as well as having a Victorian past.

As St. Louis entered the 20th century, it was the 4th largest city in the United States. Two major events held in this period, the 1904 World's Fair and 1904 Olympic Games, the first ever held in the United States, are of particular pride to St. Louisans. In the 21st century, St. Louis has transformed from a manufacturing and industrial economy into a globally known focus for research in medicine, biotechnology, and other sciences. The St. Louis region is home to 21 Fortune 1000 companies, nine of which are in the Fortune 500. The region is also home to some of the country's largest privately held corporations, including Enterprise Rent-A-Car and Edward Jones, among others.

The city has many nicknames, the most popular being "Gateway City", as it is seen as the Eastern/Western US dividing mark. St. Louis is also called "Gateway to the West" on behalf of the many people who migrated west through St. Louis via the Missouri River (first leg of the Oregon Trail) and other wagon trails. The most popular abbreviation for St. Louis is "STL" in reference to the airport code for the city and the long-standing use of an interlocked S, T and L by the St. Louis Cardinals baseball team (the St. Louis Browns also used an interlocked STL). Many natives today refer to the city as "The STL".

St. Louis lies at the heart of Greater St. Louis, a sprawling region of nearly three million people in both Missouri and Illinois. The Illinois portion is commonly known as the Metro-East. The Greater St. Louis area was the 16th largest metro area in the U.S. as of the July 2007 US Census estimate, with more than 2,800,000 people. The city itself is the 52nd largest city by population in the 2000 census and had an estimated population of 353,837 in 2006.