Brahms is one of the most complex composers of his era; one of his biographers says it is "difficult to link the master contrapuntist and variation writer with the composer of waltzes and Hungarian dances." He loved folksong and introduced it into his work from the beginning, and he also took an interest in musicology. Many music theorists consider that he fused elements of the Baroque (he was the most contrapuntal composer of the nineteenth century) and Classical eras (he adhered to Classical structure, something many Romantic composers eschewed) with the Romanticism of his day, particularly musical expressiveness. In addition to composing, he was a noted pianist, conductor, editor, scholar, collector, and critic.

His output included both sacred and secular music. Among notable compositions are his four symphonies, two piano concertos, a violin concerto, a double concerto for violin and 'cello, many chamber works, numerous piano compositions, his German Requiem, other choral works, and over 200 songs in the tradition of the German Lied. He was reluctant to discuss his own work, reflecting not only his essentially private nature but also his distrust of contemporary attitudes regarding the future of music.

Did Brahms live up to Schumann's prediction that he would "provoke the greatest upheaval within the musical world?" Two assessments provide illumination. Composer Edward Elgar said Brahms was "the classical composer par excellence of the present day, who . . . writes for the whole world and for all time-a giant, lofty and unapproachable." Musicologist J. Peter Burkholder believes Brahms is "the most imitated composer of the late nineteenth century [and] the single most important influence on twentieth-century classical music."