The purpose of this paper is to discuss the influence of playing surfaces on the forces and moments acting on the human body and the injuries they may cause for 2 selected sports activities, American football and tennis. The review is based on data from the literature and from our own investigations. A review of the effect of sports surfaces on injuries in American football leads to the conclusion that surfaces with artificial turf produce non-severe injuries more frequently than surfaces with natural grass. However, severe injuries seem to occur as frequently on natural grass as on artificial turf. It has been speculated that the shoe-surface combination which determines the frictional forces is connected with the injury frequency, i.e. the higher the frictional resistance the higher the injury frequency. Tennis surfaces have been shown to influence the occurrence and frequency of tennis injuries dramatically. The injury frequency on 'clay' and 'synthetic sand' is significantly lower than on other selected artificial surfaces. It is speculated that the differences in injury frequency are directly related to the differences in the frictional properties of the surfaces. Surfaces with low frictional resistance are assumed to cause fewer injuries than surfaces with high frictional resistance. In general, it can be concluded that the frictional property of a surface is one of the main factors to be considered when studying the aetiology of acute and/or chronic pain and injury in sports. Compliance or stiffness of surfaces, surprisingly, could not be related to the frequency of injuries on particular playing surfaces. However, it is speculated that compliance properties of surfaces are a factor which must be considered when studying chronic injuries.