The thesis is based upon a major and hitherto almost unresearched historical manuscript source, the Gardie Papers; it assesses their usefulness to the historian and the historical geographer, compares the evidence from this source with that from the extensive published literature on the Shetland Islands, and analyses data from Gardie that is not available from other sources. The first (historical) part of the work (chapters 1, 2 and 3) discusses the Garth and Gardie estates in the general context of seventeenth and eighteenth century Shetland, and the role of the Mouat family in the social, economic and political affairs of the time. The second (thematic) half (chapters 4, 5 and 6) is based on statistical analyses of data from Gardie and elsewhere; it covers a range of topics under the broad headings of 'The Estate and its Produce', 'The Tenants and the Land' and 'Problems of Demography and Labour Supply'.