This paper is a kind of supplement to Hoerner's book “Fluid Dynamic Drag.” It has a twofold purpose: to update knowledge about the effects of roughness on drag, and to clarify the aerodynamic processes by which it causes drag. The important basic flow process is shown to be one of separated flow at low Reynolds number. From this fundamental situation certain conclusions are drawn regarding transition, scale effect, permissible size of roughness, and the like. Specific requirements for aerodynamically smooth surfaces are presented and Nash's drag magnification factor for discrete roughness is considered. General procedures for estimating the drag increment due to roughness for a complete airplane are discussed. Several charts are included to guide and assist in calculation. The paper concludes with a bibliography of pertinent material beyond that mentioned by Hoerner.