The internal unity of the themes in a sonata-allegro movement and the external unity of the movements in a sonata cycle are crucial elements of Beethoven's compositional aesthetic. Numerous theorists have explored these aspects in Beethoven's sonatas, symphonies, quartets, and concertos. Similar research into the independent variation sets for piano, excluding Opus 120, has been largely neglected as the result of three misconceptions: that the variation sets, many of which were based on popular melodies of Beethoven's time, are not as worthy of study as his other works; that the type of hidden internal relationships which pervade the sonata cycle are not relevant to the variation set since all variations are, by definition, related to the theme; and that variations were composed "additively," that is, one after another, without any particular regard for their order or relationship to one another. The purpose of this study is to refute all three of these incorrect assumptions. Beethoven was concerned with the order of variations and their relationship to one another, and he was able to transcend the additive tendency in a number of ways. Some of his methods included registral connection, registral expansion, rhythmic acceleration, textural expansion, dynamics, articulation, and motivic similarities. Chapter I contains a discussion of the role of the variation set in Beethoven's overall output. The teachers, composers, and works which may have influenced him are also discussed as well as his training in variation composition. Finally, those factors which Beethoven employed to unify his sets are listed and explained. Chapters II-V are devoted to detailed analyses of four striking variation sets: Opus 34, Opus 35, WoO 80, and Opus 120. Chapter VI presents a summary of the findings. It suggests that each of the sets investigated has a unique form and that each variation has a distinct place and purpose.