Duplex stainless steels are being increasingly used by the oil and gas industry because of their high strength and good corrosion resistance. As new fields are opened up the temperatures and pressures tend to be higher and this calls for increased wall thickness even with duplex stainless steels. Because of this demand specifications increasingly demand thick sections without realizing what is needed to deliver these thicknesses. Failure to recognise this results in microstructures containing intermetallic phases (sigma chi alpha prime) or nitrides. All of these reduce toughness and corrosion resistance. In addition they can increase the risk of HISC for cathodically protected subsea components. The present paper examines the parameters that control the final structure and how these limit the maximum thickness that is practical. The optimum practices to realise the maximum thickness are discussed and case histories are presented showing both good and bad practice and the technical consequences of the latter.