The main objective of this paper is to provide information to structural engineers on how to consider and mitigate the phenomenon of pounding between adjacent structures during earthquakes. The first part of the paper reviews the problem of seismic pounding; observations of damage from pounding during recent earthquakes are described as well as the pounding potential for buildings in large Canadian cities. A review of literature on analytical and numerical studies of earthquake pounding is presented along with methods that have been proposed to mitigate pounding. In the second part of the paper, the seismic behaviour of three closely spaced, Canadian code designed, plane steel frames is investigated. Nonlinear time-step dynamic analyses and nonlinear elastic gap elements, to model pounding, are used for this purpose. Several parameters, such as separation distances, pounding locations, and ground motion characteristics, are considered. Key words: dynamics, earthquakes, impacts, pounding, seismic.