The field of high performance computing is undergoing a lot of changes. At the low end, supercomputers are becoming ever more affordable while, at the high end, it is becoming increasingly difficult to exploit the full potential of the most advanced hardware, given the sheer scale of the software challenges involved. There are also signs that the boundary between the worlds of the supercomputer and the desktop - traditionally very distinct - is starting to blur. Supercomputers continue to scale, with performance advancing at a rate that exceeds the rate of chip-level improvements, as characterized by Moore's law. These advances largely stem from the dramatic growth in cluster technology, which allows many processors to be closely coupled to solve large computational problems.