Klaus Mertens works with many important names in "Early Music", as it is generally known - specialists like Ton Koopman, Frans Brüggen, Nicholas McGegan, Philippe Herreweghe, René Jacobs, Sigiswald Kuijken, Gustav Leonhardt and Nikolaus Harnoncourt. He also works with many well-known classical conductors - Gary Bertini, Herbert Blomstedt and Sir Roger Norrington, for example.
This has resulted in some very successful work with famous orchestras, including the Amsterdam Baroque Orchestra, the Concertgebouw Orchester Amsterdam, the Rotterdam Philharmonic Orchestra, all the big Berlin orchestras, as well as Leipzig Gewandhaus Orchestra, the Dresden Philharmonic, Zurich’s Tonhalle Orchestra, the Jerusalem Symphony Orchestra, the Saint Louis Symphony Orchestra, the Chicago Symphony Orchestra and the Tokyo Metropolitan Symphony Orchestra.
Klaus Mertens is very well known and greatly in demand, particularly as an interpreter of the Baroque oratorio. He has repeatedly made recordings of Bach’s main vocal works with different conductors. In October 2003, he completed a recording of Bach’s entire cantatas. This was a ten-year project connected with extensive concert tours in Europe, America and Japan and marked a high point in his singing career. This was the first time ever that a singer had recorded the entire vocal works of Johann Sebastian Bach and also performed them in concert.