Gustav Mahler (7 July 1860 – 18 May 1911) was a Bohemian-Austrian composer and conductor.

Mahler was best known during his own lifetime as one of the leading orchestral and operatic conductors of the day, but he has since come to be acknowledged as among the most important post-romantic composers – a remarkable feat for a figure whose mature creativity was concentrated in just two genres: song and symphony. Besides the nine completed symphonies, his principal works are the song cycles Lieder eines fahrenden Gesellen (usually rendered as 'Songs of a Wayfarer', but literally 'Songs of a Travelling Journeyman') and Kindertotenlieder ('Songs on the Death of Children'), and the synthesis of symphony and song cycle that is Das Lied von der Erde ('The Song of the Earth').

Mahler told fellow composer Jean Sibelius in 1907 that "a symphony should be like the world: it must embrace everything"; putting this philosophy into practice, he brought the genre to a new level of artistic development. Increasing the range of contrasts within and between movements necessitated an expansion of scale and scope (at around 95 minutes, his six-movement Symphony No. 3 is the longest in the general symphonic repertoire) – while the admission of vocal and choral elements (with texts drawn from folk-poetry, Nietzsche, Goethe, Chinese literature, and Medieval Roman Catholic mysticism) made manifest a philosophical as well as autobiographical content. Neglected for several decades after his death, Mahler's symphonies and orchestral songs are now part of the core repertoire of major symphony orchestras.