Lester M. Petrie, Jr., is a distinguished research and development staff member in the Radiation Transport and Criticality (RTC) Group in the Nuclear Science and Technology Division. He received a B.S. degree in chemical engineering from Georgia Institute of Technology in 1959 and M.S. and Sc.D. degrees from the Massachusetts Institute of Technology in 1962 and 1965, respectively. His Sc.D. dissertation research focused on gamma ray spectra in a fusion blanket mockup. His M.S. thesis research involved degradation of an organic coolant in a reactor. Following graduate school, Dr. Petrie spent 2 years in the U.S.Army at the Nuclear Defense Laboratory, Edgewood Arsenal, Maryland, as a member of the Chemical Corps. He came to work for Union Carbide as a member of the Central Staff in 1967, was transferred to the Math Division of ORNL in 1971, and has been involved in a variety of research and analysis activities, including neutral particle transport and the physics of criticality safety. Dr. Petrie served on the DOE Language Working Group from 1978 to 1984. This group developed extensions for DOE to the Fortran 77 language standard, most of which were later adopted as part of the Fortran 90 standard. He was in charge of the shielding analysis for the CRBR shipping cask until the project was canceled. He was a member of the Organization for Economic Cooperation and Development Criticality Calculations Working Group from 1986 to 1990. Studies while on this group demonstrated limitations in the SCALE system at that time, and eventually lead to the development of CENTRM. He has been in charge of the development of the KENO series of Monte Carlo codes for determining criticality and also helped guide and implement the SCALE computer code system developed for the Nuclear Regulatory Commission to aid in license evaluations and has extensive experience in the verification and validation of computer software and data. Dr. Petrie is currently a member of the American Nuclear Society 8.12 working group for the American National Standard on out-of-reactor criticality safety and control of plutonium-uranium fuel mixtures.