  The development of the Parallel ROOT Facility, PROOF, enables a physicist to
analyze and understand much larger data sets on a shorter time scale. It makes
use of the inherent parallelism in event data and implements an architecture
that optimizes I/O and CPU utilization in heterogeneous clusters with
distributed storage. The system provides transparent and interactive access to
gigabytes today. Being part of the ROOT framework PROOF inherits the benefits
of a performant object storage system and a wealth of statistical and
visualization tools. This paper describes the key principles of the PROOF
architecture and the implementation of the system. We will illustrate its
features using a simple example and present measurements of the scalability of
the system. Finally we will discuss how PROOF can be interfaced and make use of
the different Grid solutions.
