Roughness is generally used for boundary layer flow control. Some traditional knowledge from the literature is first presented and then recent progress on roughness control is explained. Traditionally, roughness was used as an approach to advance flow transition, however some recent work has found an interesting result that roughness could delay flow transition and thus reduce the friction drag. Roughness also influences the roadmap of bypass transition and the coherent structures in the turbulent boundary layer. Since roughness can be made to form very simple configurations and can be easily attached to the object surface, such findings suggest significant potential applications of roughness in engineering.