The Grandes études de Paganini, S. 141, are a series of six études for the piano by Franz Liszt, revised in 1851 from an earlier version (published as Études d'exécution transcendante d'après Paganini, S. 140, in 1838). It is almost exclusively in the final version that these pieces are played today. The pieces are all based on the compositions of Niccolò Paganini for violin, and are among the most technically demanding pieces in the piano literature (especially the original versions, before Liszt revised them, thinning the textures and removing some of the more outrageous technical difficulties). The pieces run the gamut of technical hurdles, and frequently require very large stretches by the performer of an eleventh (although all stretches greater than a tenth were removed from the revised versions). (Source)