He has been singled out as one of the most acclaimed lyric tenors on the international stage. In opera he has performed with most of the major companies and opera houses in Europe including the Royal Opera House, Covent Garden, Deutsche Oper Berlin, the Vienna Staatsoper, Bavarian State Opera, the Salzburg Festival, Hamburg, Geneva, Madrid, Lyon and Brussels as well as the New York City Opera and the operas of St Louis, Santa Fe, Washington DC.

A consummate soloist, he often sings in America with such orchestras as the New York Philharmonic, Cleveland and Philadelphia Orchestras, and the Boston, Chicago and Los Angeles Symphonies. In Europe he has sung with the Berlin Philharmonic, Leipzig Gewandhaus, L'Orchestre Nationale de France, the BBC Symphony Orchestra and the London Sinfonietta among others, with such conductors as Barenboim, Dutoit, Gardiner, Leinsdorf, Masur, Mehta, Norrington, Ozawa, Rattle, Salonen, Slatkin, and Zinman, to name just a few.

He has sung the role of David in Die Meistersinger under James Conlon with the Gürzenich Orchestra, The Dream of Gerontius in London's Barbican Centre with Richard Hickox and the Bournemouth Symphony, followed by several Schubert works with the Chicago Symphony under Barenboim, Judas Maccabaeus with the University of Maryland Handel Festival. He has appeared in Toyko with the NHK Symphony under Charles Dutoit in Honegger's Jeanne d'Arc au Bücher and also in Leipzig in Beethoven's Ninth under Kurt Masur in the famous New Year Concerts in that city.

He has sung in recital in the Lincoln Center Vocal Series in Alice Tully Hall in New York and Philadelphia and performs with the Chamber Music Society of Licoln Center. Aler also appeared with the Boston Symphony under Ozawa in Berlioz's La Mort d'Orphée and Te Deum as well as in works by Bruckner and Mendelssohn with the Cincinnati Symphony under Lopez-Cobos and the Berlioz Requiem with the National Symphony under Slatkin.