Simon White has a towering reputation in the area of N-body simulations of hierarchical cosmology, namely the study of the formation of large scale structures as well as individual massive haloes and galaxies in the expanding Universe. The N-body simulations conducted by White since the early 70s have always been state-of-the-art, ranging from N=250, in the very earliest days, to N=10 billion recently. White has used these simulations with skill to extract what is interesting and essential, ranging from the early demonstration of a clustering hierarchy, through the development of the basic ideas and successes of cold dark matter, to the NFW "universal" halo density profile (Navarro, Frenk & White). Of this last achievement, Tremaine writes "White is probably the leading contributor to this solution."
These achievements are the highlights of many, broad, dynamical results. In addition, as a senior cosmologist, he has been a prolific and successful mentor of good students and post-docs, many of whom are today notable dynamicists in their own right.