Professor Williams is one of the pioneers of modern atomic physics. He has made very significant experimental studies of atomic structure and scattering phenomena at the most fundamental levels. Many of the contributions have been the first, and in some cases are the only measurements of fundamental atomic scattering quantities. The experimental approaches have led to new techniques in atomic physics and to an increased understanding of the mechanisms of various collision processes, indicating the ways in which electron correlations determine atomic structure and scattering dynamics.