This thesis examines the rights of landowners in Scotland in relation to water flowing through their land. In the first part of the thesis, it is argued that water is a communal thing which is incapable of ownership (or other real rights) in its natural state. Instead, the only right which anyone can have, and which everyone indeed does have, to water in its natural state is the right to obtain ownership through appropriation. In practice, however, those who own the beds of bodies of water such as rivers and lochs have the best opportunity to use water and exercise the right of appropriation due to their ability to access water freely. The second part of the thesis then examines who owns the land beneath water including the sea-bed, foreshore and alveus of rivers and lochs. The law regarding changes to boundaries between dry land and land covered by water is also investigated. The third and most substantial part of the thesis analyses the restrictions to which owners of the banks and beds of rivers and lochs are subject through common interest. This doctrine comprises a set of (generally) reciprocal rights and obligations which regulates the use of water by landowners. Common interest evolved as a result of the burgeoning use of water power between 1730 and 1830. Due to the limited material available from Roman law and the institutional writers, the courts had to experiment with various theories to resolve the disputes with which they were confronted. Following the establishment of the doctrinal foundation of common interest – in which Lord Kames had a pivotal role – there was rapid development of the content of the rights and obligations of landowners with respect to water over the 18th and 19th centuries. In light of the preceding explanation of the historical background, the final chapter of the thesis provides a restatement of the modern law of common interest. This thesis has adopted a historical and, to a lesser extent, comparative approach with the aim of providing a comprehensive study of a distinct area of water law and water rights in Scotland.