“Here’s a sonata that will challenge pianists and that one will be able to play in 50 years”: thus Beethoven is said to have written to his publisher Artaria regarding his mighty “Hammerklavier” Sonata op. 106. Given its monumental scale and extreme musical and pianistic demands, the term “Grande Sonate” seems almost an understatement. Dedicated to Archduke Rudolph of Austria, this sonata is famous not least for the seemingly unplayable autograph metronome marks in the opening movement and final fugue – not the only matter on which the editor offers new explanation. Nothing is uncomplicated in this pinnacle of the classical-romantic piano sonata, composed in “adverse circumstances” from 1817 to 1819. Even the sources pose supreme editorial challenges.