The scientific achievements of Isaiah Shavitt are reviewed. These start with the Boys and Shavitt 1956 publications on the calculation of virial coefficients and the H-3 potential surface, two of the first applications of the newly available computer to chemistry. The list of landmark achievements also includes the Gaussian transform method for calculating multicenter integrals of Slater-type orbitals and the first proposal of contracted Gaussians as basis orbitals. Shortly before Shavitt left Israel in 1967, he began a long string of seminal contributions to the method of configuration interaction, including schemes for automatic generation of lists of configuration state functions and for selecting the configurations with the largest contributions to the total energy. With these methods, he carried out extensive (for the time) configuration interaction calculations using Slater orbitals and minimum basis sets. In subsequent years at Battelle and Ohio State University, with the help of a notable group of postdoctoral fellows and graduate students, Shavitt produced, with ever-increasing accuracy, many carefully done and detailed molecular studies, including C6H6, CH2, and the H2O vibrational surface. Even more influential were his contributions to developing new techniques, particularly obtaining eigenvalues of large matrices and developing the graphical unitary group approach to configuration interaction and related correlation energy methods. The ideas originated by Shavitt throughout his career influence deeply the way we do quantum chemistry to this day. In all his work, Shavitt was noted for the thorough and careful computational procedures he insisted upon before a paper was considered complete. Isaiah Shavitt was a professor at the Technion from 1962 to 1967. In 1967 he moved to Columbus, Ohio, to take a senior research position at Battelle Memorial Institute. The following year he added a part-time faculty position at the Department of Chemistry at Ohio State University. In 1981, he withdrew from Battelle and became a full-time faculty member at Ohio State University. In 1994 he retired from this position and is continuing part-time in emeritus status.