High-performance distributed computing appears to be shifting away from tightly-connected supercomputers to "computational grids" composed of heterogeneous systems of networks, computers, storage devices, and various other devices that collectively act as a single geographically distributed "virtual" computer. One of the great challenges for this environment is providing efficient parallel data access to remote distributed datasets. In this paper, we discuss some of the issues associated with parallel I/O and computatational grids and describe the design of a flexible parallel file system that allows the application to control the behavior and functionality of virtually all aspects of the file system.