The tuba is the lowest pitched (and largest) instrument in the brass family. It is also one of the most recent instruments to make its way into the modern orchestra, as it was invented in the mid 19th century, largely replacing the ophicleide. Nowadays most orchestras have a single tuba, though a second may be exceptionally called in to perform special works. Usually the tuba plays the role of the bass of the brass section, and less frequently it is used as a solo instrument (it is a surprisingly agile instrument for its size).
Tubas can be found in various pitches, the most common being C, F, Bb, and Eb. Besides their use in orchestras, they can be also found in jazz ensembles and marching bands (where they are simply called 'basses').