With the development of inexpensive storage devices, space usage is no longer a bottleneck for computer users. However, the increasingly large amount of personal information poses a critical problem to those users: traditional file organization in hierarchical directories may not be suited to the effective management of personal information because it ignores the semantic associations therein and bears no connection with the applications that users will run. To address such limitations, we present our vision of a semantic desktop, which relies on the use of ontologies to annotate and organize data and on the concept of personal information application (PIA), which is associated with a user’s task. The PIA designer is the tool that is provided for building a variety of PIAs consisting of views (e.g., text, list, table, graph), which are spatially arranged and display interrelated fragments of the overall personal information. The semantic organization of the data follows a layered architecture that models separately the personal information, the domain data, and the application data. The network of concepts that ensues from extensive annotation and explicit associations lends itself well to rich browsing capabilities and to the formulation of expressive database-like queries. These queries are also the basis for the interaction among views of the PIAs in the same desktop or in networked desktops. In the latter case, the concept of desktop service provides for a semantic platform for the integration of information across different desktops and the web. In this paper, we present in detail the semantic organization of the information, the overall system architecture and implementation aspects, queries and their processing, PIAs and the PIA designer, including usability studies on the designer, and the concepts of semantic navigation in a desktop and of interoperation in a network of desktops.