Ludwig van Beethoven is the first name people think of when asked a question related to classical music. His symphonies, piano sonatas and choral works are still performed today, especially his symphonies. Beethoven's symphonies are well known to everyone, a good example is his Symphony No. 5 'knock-knock'. The rhythmic pattern and leap of the minor third could be hummed and recognized by many. However, few people know of Beethoven's masses, the Mass in C major Opus 86 and the Mass in D, commonly known as Missa Solemnis, Opus 124. These two masses are significant because they were written at two different times in Beethoven's life. The Mass in C was written in 1807 and was his first mass written in the traditional style (Solomon 205). The Missa Solemnis was written in 1818 and was not finished until 1823 because Beethoven spent more time playing with it until he felt satisfied (Schauffler 381). Both masses were written in the same period as his famous symphonies, respectively no. 5 and no. 9. These two masses, however, are not significant only because they were written at two different moments in Beethoven's life; furthermore they were written in two different periods of music history. The Mass in C was written towards the late Classical period, which lasted from 1750 to 1820. The Missa Solemnis was written towards the beginning of the Romantic period, which lasted from 1810 to 1914. Beethoven was faced with two different musical periods. and he made a successful career in both and by analyzing and comparing both masses, one can see the changes that Beethoven went through. This research will be divided into three sections, the Mass in C, the Mass in D, and then a comparison of Both. Each beginning of each mass will contain the story……half of the sheet……43. Book.Forbes, Elliot. Life of Beethoven by Thayer. vol. 1. Princeton: Princeton University Press, 1967. Book.—. Life of Beethoven by Thayer. vol. 2. Princeton: Princeton University Press, 1967. Book. Kerman, Joseph and Alan Tyson. The New Beethoven Grove. New York: W. W. Norton & Company, 1983. Book.Newman, Ernest. The unconscious Beethoven. New York: Alfred A Knof, 1927. Book.Rosen, Charles. The classical style Haydn Mozart, Beethoven. New York: W. W. Norton & Company, 1997. Book.Schauffler, Robert Haven. Beethoven, the man who liberated music. New York: Tudor Publishing Co., 1929. Book.Scherman, Thomas K. and Louis Biancolli. Beethoven's companion. New York: Doubleday & Company, Inc., 1972. Book.Schonberg, Harold C. The Lives of the Great Composers. New York: W. W. Norton & Company, 1997. Book.Solomon, Maynard. Beethoven. New York: Schirmer Books, 1977. Book.