Leonard Susskind, the Felix Bloch Professor in Physics at Stanford University, has made pioneering contributions to some of the most important areas in contemporary theoretical physics. Thirty-five years ago, he developed the light cone frame as a tool for the study of relativistic quantum mechanics. In 1969, with Yoichira Nambu, he proposed a revolutionary, still mysterious idea called “string theory,” which holds that the building blocks of the universe are not point-like particles, the familiar electrons and quarks, but unimaginably small, vibrating strings of some unknown component. Dr. Susskind has continued to refine his string hypothesis, now generally accepted as the only viable candidate to reconcile the differences between gravity and quantum mechanics despite its challenges to traditional understanding of space and time as well as energy and matter. Building on the idea that the universe can be compared to a hologram, in which information for a three-dimensional image can be stored on a flat surface, he discovered a matrix model as a more basic starting point for string theory. But he has also questioned string theory’s applicability to cosmology if recent observations of an accelerating universe prove true. In 2002, he suggested that an ever more rapidly expanding universe is destined to repeat itself, but that the chances it would generate a world like ours are infinitesimal. The deep paradox he suggested is that either space is not accelerating for the reasons that scientist expect or we have yet to discover some other law of nature that is responsible for cosmic evolution. A graduate of City College of New York, Dr. Susskind earned his Ph.D. in physics at Cornell University in 1965. He held a National Science Foundation post-doctoral fellowship there the next year and in 1966 joined the faculty of Belfer Graduate School of Science at Yeshiva University. Named a professor in 1970, he spent a year as a visiting professor at the University of Tel Aviv and went to Stanford in 1979. Dr. Susskind has been a Morris Loeb Lecturer at Harvard University and a Welch Lecturer at the University of Toronto. The recipient of the Pregel Award of the New York Academy of Sciences and the J. J. Sakurai Prize in Theoretical Physics, he also won a Science Writing Award given by the American Institute of Physics. He is a member of the National Academy of Sciences and the American Academy of Arts and Sciences. Dr. Susskind has published some two hundred papers in scientific journals.