Heinz Karl "Nali" Gruber (born 3 January 1943), who styles himself HK Gruber professionally, is an Austrian composer, conductor, double bass player and singer. He is a leading figure of the so-called Third Viennese School.

Gruber is said to be a descendant (though the descent remains obscure) of Franz Xaver Gruber, composer of the carol Stille Nacht (Silent Night). He was born in Vienna. From 1953 to 1957 Gruber was a member of the Vienna Boys' Choir, acquiring his nickname 'Nali' (from his snoring, he believes).^[1] He studied at the Vienna Hochschule für Musik, his composition teachers being Alfred Uhl, Erwin Ratz and Hanns Jelinek, and later Gottfried von Einem, with whom he also studied privately. In 1961 Gruber joined the ensemble die reihe as a double bass player, and became principal bass of the Vienna Tonkünstler Orchestra in 1963. In 1968, with his composer friends Kurt Schwertsik and Otto M. Zykan and the violinist Ernst Kovacic, he co-founded the 'MOB-art & tone-ART' ensemble, partly to perform their own repertoire (which included a short piece by Gruber, Bossa Nova, which rapidly became a hit tune) and partly that of Mauricio Kagel. The ensemble may be regarded as the cradle of what has been called the 'Third Viennese School', of which Gruber is now the best-known representative.

Like Schwertsik, Gruber had been taught in the post-Schoenbergian style of the Second Viennese School, but – also like Schwertsik – rapidly came to his own personal accommodation to tonality and older Viennese traditions. The critic Paul Driver has written of Gruber: ‘Neo-romantic, neo-tonal, neo-expressionistic, neo-Viennese: he isn’t any of those things, so much as a sentient (and downright accomplished) composer who keeps responding to whatever musical stimulus, be it highbrow or lowbrow, 12-tone or 7-tone, bitter or sweet, that comes his way’.

Gruber had been composing – and also playing jazz – from his student days, but achieved international fame in 1978 with Frankenstein!!, a 'pan-demonium' for chansonnier and orchestra (or large ensemble) on poems from allerleirausch, a collection of children's verse by his friend, the absurdist and Viennese-dialect poet H. C. Artmann, which he performed as singer around the world in the following few years. He and Schwertsik shared a 'Composers' Portrait' feature at the 1979 Berlin Festival, and Gruber has subsequently been ranked among Austria's leading composers. As a performer (conductor, singer, bass player) he has been involved in music by Peter Maxwell Davies, Hanns Eisler, and Kurt Weill, and made notable CD recordings of the latter two composers.

In September 2009 Gruber was appointed composer/conductor of the BBC Philharmonic Orchestra in succession to James MacMillan.^[1]