A taxonomy of applications is developed using the concept of function maps. The importance of roughness in contact mechanics is emphasised and alternative models of the plasticity index are revisited. The concept of contact resistance is shown to be strongly dependent on roughness. Further discussions are illustrated by a selection of case studies. Case studies from the life sciences include a discussion of dental and femoral prosthetics and architectural haptics. Tribological case studies are based on automotive problems, including manufacture of body parts, cylinder bore friction and lubrication, rocker cam wear and gear lubrication. It is concluded that the future of roughness studies lies with applications; that only a small number of currently available roughness parameters appear to have any practical use; and that much current work may eventually made obsolete by the increasing development of structured surfaces.