In this third of three short papers, I introduce some of the basic concepts of planetary rovers with an emphasis on some specific challenging areas of research that are peculiar to planetary robotics and not usually associated with terrestrial mobile robotics. The style of these short papers is pedagogical and this paper stresses the issue of rover-terrain interaction as an important consideration. Soil-vehicle interaction originates from military vehicle research but may be regarded as part of the dynamical approach to mobile robotics. For hostile planetary surfaces, this is essential in order to design a robotic rover with sufficient tractive capability to traverse planetary surfaces.