The parallel ROOT facility, PROOF, enables the interactive analysis of distributed data sets in a transparent way. It exploits the inherent parallelism in data sets of uncorrelated events via a multi-tier architecture that optimizes I/O and CPU utilization in heterogeneous clusters with distributed storage. On a grid, PROOF can use the available services to find out the location of data to analyze and the resources to use. Dedicated PROOF-enabled test-beds are now being deployed for testing by the LHC experiments. Being part of the ROOT framework PROOF inherits the benefits of a high performance object-oriented storage system and a wealth of statistical and visualization tools. The basic ideas and architecture underlying the PROOF project are described, together with the status of the project, focusing mostly on the issues of data access optimization, scheduling of user sessions and user interface