Despite increasingly distributed Internet information sources with diverse storage formats and access-control constraints, most of the end applications (e.g., filters and media players) that view and manipulate data from these sources operate against a traditional file-based interface. These legacy applications need to be rewritten to access remote sources, or need to rely upon ad hoc intermediary applications that aggregate the data into a passive file before executing the legacy application.This paper presents a simple, elegant, programmable method for allowing natural integration of legacy applications into distributed system infrastructures. The approach called active files enables multiple information sources to be encapsulated as a local file that serves as their logical proxy. This local file is accessed though a sentinel process, which automatically starts when the file is opened, aggregates data from multiple sources, and filters all access to and from the file. More importantly, the integration of active files into client applications is transparent: an active file is virtually indistinguishable from a regular file. Active files find a variety of applications in both distributed and non-distributed systems. We discuss active files, their semantics, their usage and their implementations in Windows NT.