This document describes the two membranes containment systems developed by the company Gaztransport & Technigaz for the transportation at sea of liquefied natural gas (LNG) and applied by several shipyards in Europe, United States, Japan and Korea. The merits of both containment systems are underlined with respect to the competition for operating flexibility as well as safety during LNG carrier operation. Finally, the merits of both containment systems in terms of construction and coasts are presented.