This thesis explores arrangements of British Isles folk song melodies as set by major composers: Franz Joseph Haydn (1732-1809), Ludwig van Beethoven (1770-1827), Ralph Vaughan Williams (1872-1958), and Benjamin Britten (1913-1976). These composers found vivid inspiration and a certain amount of success by setting folk songs as art music, but a survey of available scholarship centering on their works in most instances reveals only a superficial discussion of their folk song arrangements. Moreover, folk song studies, which tend to focus on the existence of melodies, their origins, and modern efforts in collection, also avoid the arrangements by serious composers. A few important studies exist which deal specifically with folk song arrangements, but by and large this area suffers from neglect. This is no doubt due to a misunderstanding of the term arrangement as it applies to the examples from the works of these composers, and its connotation as a composition of lesser value than wholly original works. This paper explores the folk song arrangements by these four composers through a study of similar melodies from their collections, which altogether number more than eight hundred songs. A review of the historical context of these collections reveals the degree of importance they occupied in each composer's overall output of works. Then an analysis of one melody set three different ways by Vaughan Williams, as well as analytical comparisons of arrangements of seven other melodies set by pairs of these four composers demonstrates the various devises each used to bring originality, creativity, and musical artistry to a work despite the limitations proscribed by a preexisting melody.