The present paper presents a rational expression for initiation of motion for non-cohesive sediment under rough turbulent conditions. Previously proposed relationships for the initiation of motion are examined as well as laboratory measurements corresponding to fully rough turbulent boundary layers (which is normally the case under prototype conditions). By combining the modified Shields criterion as proposed by Bagnold (1963) with the wave friction factor, fw, from Jonsson (1966), an easily applied criterion for the threshold velocity for incipient motion under rough turbulent conditions is derived. Predictions with the new criterion are shown to be in good agreement with measurements over the full range of typically encountered engineering conditions.