Piano Sonata No. 21, also known as the Waldstein Sonata, is a piano sonata composed by Ludwig van Beethoven in 1804. It is one of his most celebrated works, showcasing his innovative style and emotional depth. The sonata consists of three movements, with the first movement being particularly notable for its energetic and dynamic character. The second movement serves as a lyrical contrast, featuring a serene and flowing theme. The final movement is marked by its lively tempo and virtuosic passages, culminating in a triumphant conclusion. Overall, the sonata reflects Beethoven's transition into his middle period, highlighting his mastery of form and expression.