Piano Sonata No. 21 in C major, Op. 53, also known as the "Waldstein Sonata," is a significant work composed by Ludwig van Beethoven in 1804. It is renowned for its innovative structure and emotional depth, showcasing Beethoven's transition from the Classical to the Romantic style. The sonata consists of three movements, with the first movement being particularly notable for its energetic themes and complex development. The second movement serves as a lyrical interlude, providing contrast before the final movement, which is marked by its lively tempo and virtuosic passages. This sonata is often celebrated for its technical demands on the pianist and its expressive qualities, making it a favorite in the classical piano repertoire.