The Moldenhauer Archives at the Library of Congress contain approximately 3,500 items documenting the history of Western music from the medieval period through the modern era.

Here is a nice introduction to the Romantic Era. It describes the various elements and the composers of this time.

Learn about the various styles of music from the 19th century. From romanticism to nationalism, this website goes into detail about the history of 19th century music.

This site contains the most significant artistic output of the most important composers within each historical period.