This thesis concerns Mary Elizabeth Braddon's and Wilkie Collins's fictional representation and employment of theories of biological and environmental determinism. It demonstrates that both authors saw determinism as a theme for literary exploration, and also used it as a means of addressing associated issues debated throughout the Victorian period such as class, gender roles and moral responsibility. The introductory first chapter provides an overview of sensation fiction and concurrent theories of determinism. Part 1 begins with a consideration of Wilkie Collins's use of early- to mid-Victorian psycho-physiological theories, in particular his depiction of monomania. Chapter 2 argues that Collins uses monomania to simultaneously explain and enhance the melodramatic atmosphere of Basil. Chapter 3 shows how his portrayal of monomania in No Name facilitates an engagement with Victorian debates about willpower and personal responsibility. Chapter 4 reveals how in 'Mad Monkton' and Armadnle, Collins's speculations about heredity anticipate, without endorsing, theories of degeneration. The final chapter on Collins discusses how in Man and Wife his artistic change in direction towards "novels with a purpose" is accompanied by a greater emphasis on environmental determinism. Part 2 begins with an analysis of Braddon's under-studied The Lady Lisle which uses ideas of nature and nurture to explore and manipulate class boundaries. Chapter 7 examines John Marchmont's Legacy's interrogation of notions of the "ideal woman", and its assertion that some women are constitutionally incapable of fulfilling such a role. The final chapter concerns Braddon's continued exploration of the connections between determinism and womanliness in the little-known Lost for Love, in which she depicts women as capable of great intellectual achievement if given the correct education. However, this depiction is filtered through a conservative ideology of gender which asserted that women should be primarily trained as companions to men.