Many of our everyday activities manifest themselves in digital form; either in an explicit way, when we actively use digital information for work or spare time; or in an implicit way, when information is indirectly created or manipulated as a consequence of our action. The storage and processing capacity of the devices we use to interact with these data has been increasing enormously over the last years, and we can expect this development to continue in the future. However, while the power of physical data storage is permanently increasing, the development of logical data organization power of personal devices has been stagnating since the invention of the first personal computers. This book presents an alternative organizational model for the management of personal data that strikes a balance between the unstructured nature of file systems and the highly formal characteristics of logic-based systems.