Orgelbau Klais is a German firm that designs, builds and restores pipe organs. It is a family run company, founded in 1882 by Johannes Klais senior and is now run by his great-grandson Philipp Klais. The firm is based in Bonn, Germany, and has completed many large-scale building and restoration projects around the globe in more than a century of organ building.^[1]

Johannes Klais studied organ building in Alsace, Switzerland and Southern Germany. He founded his own organ building workshop in Bonn in 1882. His way of building organs was closely bound up with traditional construction methods using slider windchests. But as early as before the turn of the century he built high pressure stops with two mouths on pneumatic cone valve chests. In 1906, together with his son Hans, he introduced electric action. Hans Klais took over in 1925. In his time facade design began to come under the influence of the modern age, ergonomic console designs were also being developed. Hans Gerd Klais, the founder's grandson, took charge in 1965. Philipp Klais, the great-grandson of the founder, studied organ building in Alsace, France; in Germany; and overseas. He now runs the company.

Here is a short list naming a few of the Klais instruments around the world:

• Athens Concert Hall, in Athens, Greece with 6080 pipes^[2]
• Aachen Cathedral in Aachen, Germany.^[3]
• Bath Abbey in Bath, Somerset, UK.^[4]
• Beuron Archabbey church, Germany^[5]
• Basilica of Our Lady del Pilar in Saragossa, Spain, with 5.391 organ pipes inserted in a Renaissance (1529-1530) wooden frame, and inaugurated in 2008
• Cologne Cathedral in Cologne, Germany: The main instrument was finished in 1998 and although enormous, it is almost dwarfed inside the colossal gothic church as it clings to the balconies. Klais also restored other organs in the cathedral and added a detached console that operates all of them at once.
• Centro Cultural Kirchner, Buenos Aires, Argentina.
• Church of Rodeio, Santa Catarina, Brazil.
• Elbphilharmonie in Hamburg, Germany: 4 manuals, 65 registers, 4,765 pipes, completed December 2017
• Esplanade Concert Hall in Singapore, 4740 pipes, 61 registers
• First Community Church, Columbus, Ohio. 4 manuals, 85 ranks. Dual consoles. 2022.
• Frauenkirche in Nuremberg, Germany
• Hallgrímskirkja in Reykjavík, Iceland, 5275 pipes.
• Kraków Philharmonic Concert Hall, in Kraków, Poland
• Leeds Cathedral, UK
• Münster Cathedral in Münster, Germany, approx. 7173 pipes
• National Centre for the Performing Arts in Beijing, China. Finished in 2007, the 6500 pipes symphony organ with 94 stops is now the largest organ in China.^[6]
• Ohio Wesleyan University, in Delaware, Ohio, U.S.: 82 Ranks 55 Stops 4,644 pipes^[7]
• Overture Hall in Madison, Wisconsin, U.S.^[8]
• Petronas Philharmonic Hall in Kuala Lumpur, Malaysia
• Queensland Performing Arts Centre, in Brisbane, Australia, 6566 pipes^[9]
• St. John's, Smith Square in Westminster, UK, a redundant but still consecrated church which commissioned its organ when the building became a concert hall.
• St. Peter's church in Munich, Germany
• St. Peter's Lutheran Church in New York City, U.S. 1978, 2 manuals, 43 ranks.^[10]
• Symphony Hall, Birmingham in Birmingham, UK. Finished in 2001, the 6000 pipes symphony organ is now the largest mechanical action organ in the UK.^[11]
• Trier Cathedral in Trier, Germany, 5602 pipes
• The University of Iowa Voxman Music Building Concert Hall in Iowa City, IA, United States, completed in 2016, 3,883 pipes.

• Choir organ in Cologne Cathedral
• Trier Cathedral (1974)
• Kraków Philharmonic
• Münster Cathedral
• Beuron Archabbey
• Liebfrauenmünster Donauwörth