In music, Op. 66 stands for Opus number 66. Compositions that are assigned this number include:
Britten – War Requiem Chopin – Fantaisie-Impromptu Dvořák – Scherzo capriccioso Mendelssohn – Piano Trio No. 2 Myaskovsky – Cello Concerto Schumann – Bilder aus Osten (Pictures from the East), 6 Impromptus for piano 4-hands Scriabin – Piano Sonata No. 8 Strauss – Der Krämerspiegel