The Social-Democratic Federation has been ill-served byhistorians, dismissed as an irrelevance or an alien intrusioninto British politics. This thesis attempts to provide abalanced and coherent account of the SDF's history, emphasisi:regional as well as national developments to demonstrate thatuntil the early years of the twentieth century, the partyposed a genuine alternative to the supposed 'mainstream'development of the ILP/Labour Party. The Federation was farfrom the monolithic, centralised organisation, dominated byHyndman, thatis often depicted. A study of the branches inLancashire and Yorkshire reveals regional diversity anddemonstrates that they enjoyed considerable autonomy, butalthough this autonomy allowed branches in areas likeLancashire to adapt to their environment with considerablesuccess it also produced a party prone to internal divisionsover strategy. Consequently it failed to develop consistentpolicies. This proved a fatal handicap at a crucial periodin the history of the British Socialist movement, during theformative years of the Labour Party. The SDF was marginalised,preoccupied with its own internal debates at a timewhen it could have exercised considerable influence insideLabour's ranks. It never satisfactorily resolved the debateover which course to pursue, that of reform or revolution,until the outbreak of the First World War brought the divisioiwithin the party to a head, which ultimately caused itsdissolution. Nevertheless its eventual demise should notobscure its achievements which, as is often the fate ofpioneers, remain largely unsung. It educated and agitated;it played a leading role in the formation of both ILP branchesand Labour Representation Committees; it produced a generationof working-class intellectuals and militants; it championedthe cause of the unemployed. Most important of all, the SDFwas responsible for re-introducing Socialism to the Britishpolitical agenda.