In-plane scatter at 0.6328 μ m was measured from a master set of four samples at 18 optical scatter measurement facilities in the United States. They included government laboratories, universities and industry. The samples were a diffuse white, diffuse black, Molybdenum mirror (45 A rms) and Aluminum mirror (7 A rms). Measured scatter at an incidence angle of 10 degrees and a forward scatter angle of 35 degrees varied by a factor of 2 on the diffuse white (mean = 0.27/sr) and a factor of 5 on the aluminum mirror (mean = 61 ppm/sr). The variations in measured scatter values are a result of inadequate calibration techniques, incorrect gain factors, incorrect optical filter factors, inadequate instrumentation or, in some cases, incorrect operating practices. As a result of this round robin study, we are now pursuing a "standard test method" for measurement of scatter. We are also putting increased emphasis on in-house scatter measurement capability, training and availability of reference samples.