Alfredo entered the Conservatoire de Paris in 1896 to study piano under Louis Diémer and composition under Gabriel Fauré; in these classes, Lazare Lévy, George Enescu and Maurice Ravel were among his fellow students.

During his Parisian period, Claude Debussy, Igor Stravinsky and Manuel de Falla were acquaintances, and he was also in contact with Ferruccio Busoni, Gustav Mahler and Richard Strauss.

Casella developed a deep admiration for Debussy's output after hearing Prélude à l'après-midi d'un faune in 1898, but pursued a more romantic vein (stemming from Strauss and Mahler) in his own writing of this period, rather than turning to impressionism.

His stature as a pianist and his work with the trio gave rise to some of his best-known compositions, including A Notte Alta, the Sonatina, Nove Pezzi, and the Six Studies, Op.

His editions of Johann Sebastian Bach and Ludwig van Beethoven's piano works, along with many others, proved extremely influential on the musical taste and performance style of Italian players in the following generations.

[3] The generazione dell'ottanta ("generation of the '80s"), including Casella himself, Malipiero, Respighi, Pizzetti, and Alfano—all composers born around 1880, the post-Puccini generation—concentrated on writing instrumental works, rather than operas, which Puccini and his musical forebears had specialised in.

[4][5] Casella, who identified for many years with the Fascist regime in Italy, found himself in conflict with it after the legislation of the Italian racial laws of 1938, his wife being Jewish of French origin.