Rolf Riesen is a principal member of technical staff at Sandia National Laboratories in Albuquerque, New Mexico. He holds a Ph.D. degree in computer science from the University of New Mexico (UNM). His research interests include message passing systems, operating systems, and runtime software for massively parallel computers. Over the last ten years in this field he has primarily concentrated on topics related to efficient, scalable message passing and interactions at the software/hardware boundary. Dr. Riesen has been a key member of the design team for SUNMOS (Sandia/UNM OS) for the nCUBE 2 and the Intel Paragon, as well as Puma/Cougar, the second generation light weight kernel for the Intel ASCI Red machine. He is also a key designer of the Portals message passing mechanism. He has been a principal designer of Cplant, the largest commodity cluster for scientific applications in the world. He was involved in the overall design and the low level message passing layers, including the scalable wire protocol used by Cplant.