In the wheel-rail system roughness, which is defined as repeated irregularity on an idealised flat wheel motion line, occurs over a broad scale from m to meters. A clearly defined overview and classification of this aspect is not present at the moment. For this reason experts speak of roughness and not rarely mean thereby completely different forms of irregularities. Aim of this paper is an extensive description of the topic roughness in the wheel-rail system as well as a distinct classification and description of these phenomena. Furthermore reasons, influencing factors, effects and remedies for the irregularities over the whole scale will be discussed and an overview of the actual measuring techniques will be given. The paper should build the basis for better understanding of the topic roughness to provide the possibility for improvements in the wheel-rail system. Due to the increasing sensitisation of the population regarding noise emission and the direct relation to irregularities, this area will be discussed separately. Relations will be presented and possible active and passive improvement measures will be highlighted. For presenting the actual relevance of the topic moreover the regulatory framework will be illustrated.