The Social-Democratic Federation has been ill-served by historians, dismissed as an irrelevance or an alien intrusion into British politics. This thesis attempts to provide a balanced and coherent account of the SDF's history, emphasisi: regional as well as national developments to demonstrate that until the early years of the twentieth century, the party posed a genuine alternative to the supposed 'mainstream' development of the ILP/Labour Party. The Federation was far from the monolithic, centralised organisation, dominated by Hyndman, thatis often depicted. A study of the branches in Lancashire and Yorkshire reveals regional diversity and demonstrates that they enjoyed considerable autonomy, but although this autonomy allowed branches in areas like Lancashire to adapt to their environment with considerable success it also produced a party prone to internal divisions over strategy. Consequently it failed to develop consistent policies. This proved a fatal handicap at a crucial period in the history of the British Socialist movement, during the formative years of the Labour Party. The SDF was marginalised, preoccupied with its own internal debates at a time when it could have exercised considerable influence inside Labour's ranks. It never satisfactorily resolved the debate over which course to pursue, that of reform or revolution, until the outbreak of the First World War brought the divisioi within the party to a head, which ultimately caused its dissolution. Nevertheless its eventual demise should not obscure its achievements which, as is often the fate of pioneers, remain largely unsung. It educated and agitated; it played a leading role in the formation of both ILP branches and Labour Representation Committees; it produced a generation of working-class intellectuals and militants; it championed the cause of the unemployed. Most important of all, the SDF was responsible for re-introducing Socialism to the British political agenda.