Carl August Nielsen (9 June 1865 – 3 October 1931), widely recognised as Denmark's greatest composer, was also a conductor and a violinist. Brought up by poor but musically talented parents on the island of Funen, he demonstrated his musical abilities at an early age. While it was some time before his works were fully appreciated, even in his home country, Nielsen has now firmly entered the international repertoire. Especially in Europe and the United States, Nielsen's music is ever more frequently performed, with interest growing in other countries too. Carl Nielsen is especially admired for his six symphonies, his Wind Quintet and his concertos for violin, flute and clarinet. In Denmark, his opera Maskarade and a considerable number of his songs have become an integral part of the national heritage. While his early music was inspired by composers such as Brahms and Grieg, he soon started to develop his own style, first experimenting with progressive tonality and later diverging even more radically from the standards of composition still common at the time. For many years, he appeared on the Danish hundred-kroner banknote.

Nielsen is best known for his six symphonies. Other well-known pieces are the incidental music for Adam Oehlenschläger's drama Aladdin, the operas Saul og David and Maskarade, the three concertos for violin, flute and clarinet, the Wind Quintet, and the Helios Overture, which depicts the passage of the sun in the sky from dawn to nightfall. Most Danes can sing many of the numerous songs by various poets, set to music by Carl Nielsen.
The music initially had a neo-classical sound but became increasingly modern as Nielsen developed his own approach to what Robert Simpson called progressive tonality, moving from one key to another. Typically, he would end on a different key, sometimes as the outcome of a struggle as in his symphonies. His frequently blended melodic passages inspired by folk music with more complicated stylings including counterpoint and modern variations.
Like his contemporary, the Finn Jean Sibelius, he studied Renaissance polyphony closely, which accounts for much of the melodic and harmonic "feel" of his music.