### Instruction: Who composed the 'Moonlight Sonata', and when?
### Response: Commonly known as the Moonlight Sonata, the Piano Sonata No. 14 in C-sharp minor "Quasi una fantasia", Op. 27, No. 2, was composed by Ludwig van Beethoven.
The composition was completed in 1801 and dedicated to the Countess Giulietta Guicciardi in 1802. It's one of the most popular musical compositions for the piano.