Experimental research on rough wall turbulent boundary layers can be traced to the historic experiments of Darcy (1858) on pipe flow. The chapter presents an extensive chronological survey of experiments on incompressible and compressible turbulent flow over rough walls for pipe and channel flow and boundary layers. The concept of the equivalent sand grain roughnes is described, together with the regimes of hydraulically smooth, transitionally rough, and fully rough flows. The relationship between the equivalent sand grain roughness and various statistical measures of surface roughness topology is described. Evidence of the validity of Townsend’s Townsend, A. similarity hypothesis is presented. Flowield statistics including mean velocity profiles, Reynolds stresses, two point velocity correlations, and quadrant analysis are described. The influence of the roughness topology on the turbulence structure is analyzed.