An overview of recent work on the problem of turbulent boundary layers developing over surface roughness will be given. This includes experimental laboratory studies, numerical simulations and recent attempts at full-scale in-situ measurements on the hull of an operating ship. The overarching aim here is to be able to make full-scale predictions of the penalty (economic / environmental / performance) resulting from surface roughness on the hulls of operating ships. This roughness could be due to the build-up of marine organisms on the hull of the ship or due to the surface finish attained during the hull coating process. For a given surface topography of interest, a key element to making these full-scale predictions is the ability to determine the equivalent roughness height (which is a measure of the degree to which the surface topography affects the flow). Several methods of estimating this roughness height will be discussed as well as a methodology for using this to obtain full scale predictions. Finally, a direct method will be presented for inferring the roughness penalty from an in-situ measurement of the boundary layer over the hull of an operating ship. .