As a tenor, professor Rieger has had great success performing opera throughout Europe, where he has sung under such conductors as John Elliot Gardner, Stefano Ranzani and Franz Welser-Möst. His career has led him to the opera companies of Zürich, Luzern and Basel (in Switzerland); Trier, Regensburg, Kaiserslautern, Bremerhaven, Osnabrück, Nordhausen and Konstanz (Germany); and Novara (Italy). He has also performed at Zomeropera Alden Biesen (Belgium), Citizens Theatre (Scotland), Everyman Palace Theatre (Ireland) and the Mozart and Friends Opera Festival (New Jersey).