This thesis examines the interaction between the conflicting ideologies of crusade and commerce, during the period when the Turkish maritime emirates of Anatolia became the primary target of western crusading endeavour. Through the close study of papal documents and archival evidence from the Italian mercantile republics, two principal areas are focussed on: firstly, the extent to which the temporal and spiritual mechanisms (e.g. trade licences and indulgences) introduced by the popes of the fourteenth century encouraged the Italian mercantile republics to participate in a crusade; secondly, the analysis of the policies of commercial exchange and military opposition adopted by the Latin states with regard to the Turks in the Aegean.