In its early years, the Guarneri's repertoire focused primarily on music of 18th and 19th century composers, and they became particularly known for their interpretations of Beethoven’s quartets. However, by the 1980s the quartet was incorporating more music from the 20th century in its programs. The musicians took a selective approach to modern musicthey were not enthusiastic about what Tree called "very experimental, avant-garde music" and they had mixed feelings about Schoenberg and Shostakovichbut they became champions of the quartets of Bartók and their repertoire eventually included music by Webern, Berg, Stravinsky, Vincent Persichetti, Paul Hindemith, Witold Lutosławski, Hans Werner Henze, Leon Kirchner, and William Klenz. Several original compositions were written for the group, by Ned Rorem, Lukas Foss, Mario Davidovsky, and Richard Danielpour. The group also made a point of performing seldom-heard quartets by earlier composers, including those of Jean Sibelius, Leoš Janáček, Zoltán Kodály, and Juan Crisóstomo de Arriaga.