An étude is a musical composition (usually short) designed to provide practice in a particular technical skill in the performance of a solo instrument.
Robert Schumann (1810–1856): Studies (Op. 3) and Études (Op. 10) after Paganini's Caprices; and the Symphonic Studies (Op. 13, in three revisions: 1834, 1852, and posthumously 1893).
Frédéric Chopin (1810–1849): 24 études in two sets of 12 each (Opp. 10 and 25), plus three more (a little easier), for a total of 27.
Franz Liszt (1811–1886): set of Transcendental Études, with its two previous versions being Étude en douze exercises and Douze Grandes Études; six études, also with an earlier set, on themes by Niccolò Paganini (among them the famous La Campanella); and six concert études (one set of three, another set of two and Ab Irato which also has an earlier version). In contrast with Chopin's études, which tend to stress a specific aspect of performance difficulty, Liszt's études tend to stress mastery of performance as a whole. Liszt also wrote 12 books of Technical Studies (S.146) between 1868 and 1880.
Charles-Valentin Alkan (1813–1888): Trois Études de bravoure (Op. 16); études in all 12 major keys (Op. 35) and in all 12 minor keys (Op. 39); and also three Grande Études (Op. 76). He also wrote three other études: Op. 17 (Le preux), Op. 27 (Le chemin de fer), and two without opus number.
Stephen Heller (1813–1888): twenty-four études Op. 16, 25 Études Op. 45, 30 Études Op. 46, 25 Études Op. 47, 24 Nouvelles études d'expression et de rhythme Op. 125, 4 Études d'après Der Freischütz Op. 127, twenty-one Études techniques pour préparer à l'exécution des ouvrages Op. 154, among others
Henrique Oswald (1852–1931): Trois Etudes pour piano, plus three independent études
Moritz Moszkowski (1854–1925): three concert studies (Op. 24), Ecole des doubles notes (Op. 64), 15 Études de Virtuositié (Op. 72), 12 studies for the left hand alone (Op. 92), and 20 technical studies (Op. 91).
Leopold Godowsky (1870–1938): sixty studies on Chopin's études, of which 53 are published; three original Concert Studies (Op. 11), and the Étude Macabre.
Alexander Scriabin (1872–1915): twenty-six études (Opp. 2, 8, 42, 49, 56 and 65)
Ezequiel Viñao (born 1960): first book of études in 1993
Marc-André Hamelin (born 1961): twelve études in minor keys, and an étude after Rimsky-Korsakov (which was the previous 1st piece of the 12 études set, but it was replaced by the Triple Étude after Chopin-Godowsky)
Lowell Liebermann (born 1961): Etudes on Brahms Songs Op.88 (2004), Etudes on Songs of Robert Franz Op.91 (2005)
Unsuk Chin (born 1961): currently working on a set of 12 Piano Studies, of which six have been completed
Darius Milhaud (1892–1974): Cinq études, Op. 63, for piano and orchestra (1920); Etude poétique, Op. 333, musique concrète (1954); Études, Op. 442, for string quartet (1973)
Olivier Messiaen (1908–1992): Vocalise-étude, for soprano and piano (1935)
Pierre Schaeffer (1910–1995): Cinq études de bruits (1948), for musique concrète (turntable technology), consisting of (1) "Étude aux chemins de fer", (2) "Étude aux tourniquets", (3) "Étude au piano I", (4) "Étude au piano II", and (5) "Étude aux casseroles". Also wrote Étude pathétique (1948), Étude violette (1948), both turntable-technology musique concrète, Étude aux sons animés (1958), Étude aux allures (1958), and Étude aux objets (1959), all for tape-technology musique concrète