A famous conductor during his lifetime, Gustav Mahler has since been deemed a significant composer whose work is a bridge between the Romanticism of the 19th century and the modern compositions of the 20th century. Gustav Mahler studied piano and composition in Vienna when he was a teenager, and by the time he was in his 20s he was making a name for himself as a conductor. He held positions in Prague, Leipzig and Budapest before becoming the chief conductor of the Hamburg Opera in 1891. He also conducted Vienna's State Opera House (1897-1907) and the New York Philharmonic Society (1908-11), while composing songs, nine numbered symphonies and, his most famous work, Das Lied von der Erde (1908, not called a symphony by Mahler because of superstition -- he admired Anton Bruckner and Ludwig Van Beethoven, both of whom died after writing nine symphonies). A generation after his death, compositions such as his Kindertotenlieder (1901-04) ("Songs on the Death of Children") became orchestral standards, known for heavy emotion and expressions of mourning.

Extra credit: Gustav Mahler was born a Jew, but became a Roman Catholic as an adult.