Custom records have long been recognised as an invaluable aid in the study of overseas trade. In Scotland these records have survived virtually intact from the middle of the fourteenth century, and their importance is magnified considerably owing to the limited number of alternative sources relating to trade. Moreover, they are the only long-term quantitative source relating to the economy of medieval and early modern Scotland. In previous work on the Scottish custom accounts little attention has been given to their reliability as export figures, while those export figures which have been presented have tended to be limited in coverage and lacking in precision. As a result, it is impossible to establish accurately either the long-term or short-term trends in Scottish overseas trade. This thesis examines the customs administration, looking at the extent and changes in the custom jurisdictions, the export locations, the incidence of smuggling and embezzlement, clerical mistakes made during the compilation of the accounts, and the costuming procedure for each major commodity- wool, woodfells, hides, cloth, fish, salt, coal, lead, re-exports, and English imports. This analysis determines the value and limitation of the custom accounts as evidence of exports. For the first time, therefore, together with consideration of these findings, export figures, in both tubular and graphical form, are presented, for each year from 1328 to 1600: export figures are given for each commodity and total trade, for each custom jurisdiction and for the whole country. Finally, with the aid of the export figures, the trends in Scottish exports are discussed, and compared with the situation that pertained in England. It is hoped that this thesis will further the study of European trade, and be a significant tool for the examination of the medieval and early modern Scottish economy.