The paper deals with the tribology of surface roughness effects in rapidly moving hydraulic units, such as pumps, motors and seals. The emphasis is on laminar flow, turbulence effects being only briefly discussed. The analysis is based on lubrication theory that was originally developed for bearings in non-hydraulic equipment. However, the established results are readily adoptable to a hydraulic environment. The effects may be quasi-static or dynamic. It is shown that surface roughness may affect the operation of hydraulic units negatively or positively, depending on the actual conditions, resulting in a need for optimisation and compromise, but also in the possibility to tailor surfaces for optimal operation. Finally, for situations when an undesirable roughness structure on the surfaces is unavoidable, the paper presents methods by which the surface finish (roughness amplitude) necessary for safe operation may be determined.