William Caballero has been Principal Horn of the Pittsburgh Symphony since 1989. Formerly he was Principal and Co-Principal Horn of the Houston Symphony, Houston Grand Opera (1985-1989), Third Horn of the Montreal Symphony, Montreal Opera (1984-1985), Acting Third Horn of the Boston Symphony Orchestra and Boston Pops (1982-1984), and Principal Horn of the Hartford Symphony (1982). Other guest Principal horn opportunities have included the Chicago Symphony Orchestra, Cleveland Orchestra, Philadelphia Orchestra, Los Angeles Philharmonic, Dallas Symphony Orchestra, and the Saint Louis Symphony Orchestra. Festivals have included the Grand Teton Music Festival, Aspen Music Festival, Bay Chamber (Maine) Concert Series, and the Pacific Music Festival in Sapporo, Japan. He has also performed chamber music with Gil Shaham, Orli Shaham, Sir Andre Previn, Christoph Eshenbach, Andre Watt, and the Tokyo String Quartet,
He is presently the Associate Teaching Professor of Horn at Carnegie Mellon University, Pittsburgh, PA. Other teaching positions have included Adjunct Teacher of Horn at Rice University, Duquesne University, and Indiana University Bloomington. Recent master classes and teaching have included Northwestern University, Indiana University (Bloomington), Maryland University, New England Conservatory, Colburn Conservatory, Beijing Conservatory, Shanghai Conservatory, and the Pacific Music Festival.