Julius Allan Greenway Harrison (26 March 1885 – 5 April 1963) was an English composer and conductor who was particularly known for his interpretation of operatic works. Born in Lower Mitton, Stourport in Worcestershire, by the age of 16 he was already an established musician. His career included a directorship of opera at the Royal Academy of Music where he was a professor of composition, a position as répétiteur at the Royal Opera House, Covent Garden, conductor for the British National Opera Company, military service as an officer in the Royal Flying Corps, and founder member and vice-president of the Elgar Society.

Harrison was born in 1885 in Lower Mitton, Stourport in Worcestershire. He was the eldest of four sons and three daughters of Walter Henry Harrison a grocer and candle maker from the village of Powick near Malvern, and his wife, Henriette Julien née Schoeller, a German-born former governess. He was educated at a dame school in Stourport, and at Queen Elizabeth's Grammar School, Hartlebury. The family was musical; Walter Harrison was conductor of the Stourport Glee Union, and Henriette was Julius's first piano teacher. He later took organ and violin lessons from the organist of Wilden parish church, and sang in the church choir.

At the age of 16 Harrison was appointed organist and choirmaster at Areley Kings Church, and at Hartlebury Church at the age of 21. When he was 17 he directed the Worcester Musical Society in a performance of his own Ballade for Strings. He gained two Firsts in music in Cambridge local examinations, and studied under Granville Bantock at the Birmingham and Midland Institute of Music where he specialised in conducting.

He first came to wider public notice in 1908 with his setting of Gerald Cumberland's cantata libretto Cleopatra. Harrison's setting won the first prize at the Norwich Musical Festival, adjudicated by Frederick Delius, Samuel Coleridge-Taylor and Ernest Walker. The Times commented on the inadequacy of the libretto, and praised Harrison's orchestration and melodies but complained that the work was "a series of pictures of unbridled passion devoid of all that ordinary people call beauty." The reviewer in The Manchester Guardian was more complimentary; though he commented on the obvious influence of Bantock, and over-elaborate orchestration, he wrote that Harrison had undoubted talent.

Moving to London when he was 23, he took a job with the Orchestrelle Company, a manufacturer of rolls for player-pianos. He conducted amateur ensembles and was organist of the Union Chapel, Islington. In the latter capacity he wrote several pieces for the choir during 1910 and 1911, and his symphonic poem Night on the Mountains was played at the Queen's Hall by the London Symphony Orchestra, conducted by Harrison at the invitation of Hans Richter. The Times said, "The orchestral colouring is laid on with so thick a brush that the outlines get somewhat obscured in places, but it still contains some promising ideas".

For most of his career Harrison was obliged to earn a living by conducting and other musical work, to the detriment of his composing. In early 1913 he was engaged as a répétiteur at Covent Garden, where he had the opportunity of observing Arthur Nikisch prepare Wagner's Der Ring des Nibelungen. Later that year Harrison was appointed to the conducting staff for the season. In 1914 he was assistant conductor to Nikisch and Felix Weingartner in Paris, rehearsing Parsifal for the former and Tristan und Isolde for the latter.

In 1915 Thomas Beecham and Robert Courtneidge presented a season of opera at the Shaftesbury Theatre. Harrison was recruited as a conductor along with Percy Pitt, Hamish MacCunn and Landon Ronald. After a second season with Courtneidge, Beecham set up on his own account in 1916, and established the Beecham Opera Company at the Aldwych Theatre of which his father Sir Joseph Beecham was the lessee. Harrison, Pitt, and Eugene Goossens joined him as assistant conductors. In 1916 Harrison joined the Royal Flying Corps and was commissioned as a lieutenant in the technical branch. He was based in London, and was frequently able to conduct for Beecham, often wearing his uniform.

From 1920 to 1923 Harrison was co-conductor of the Scottish Orchestra with Ronald, and from 1920 to 1927 he was also in charge of the Bradford Permanent Orchestra. From 1922 to 1924 he was a conductor for the British National Opera Company, specialising in Wagner.