The Requiem in D Minor, K. 626, is a piece of music by Wolfgang Amadeus Mozart which he composed in 1791. This requiem (a 'requiem' is a mass (church service) to remember people who have died), was Mozart's last composition. He wrote it as he lay dying. It seems that he did not manage to finish it before he died. It was finished by Franz Xaver Süssmayr, another composer. We do not know exactly how much of the music is by Mozart, and how much Süssmayr had to add. There have been lots of stories and ideas about how the Requiem was composed, and it is still a problem that is often discussed by musicians.