The invention relates to a tank for storing fluid. In one embodiment, a tank (10) for storing fluids is disclosed, the tank comprising a vessel (12) for receiving and storing fluid, and a reinforcing structure (14) for the vessel. The reinforcing structure is located externally of the vessel. The vessel has a generally quadrilateral shape in plan view, and a base (16) which is generally dish-shaped, with an outlet (18) located in a position which facilitates discharge of the fluid from the vessel. In providing a tank having a vessel which is generally quadrilateral shape in plan view, the tank offers benefits over prior cylindrical tanks in terms of maximizing the use of available storage space on a ship, or on a vehicle used to transport the tank. Additionally, in providing a tank having a vessel which has a base that is generally dish-shaped, the tank offers improvements over prior quadrilateral shape tanks. In particular, the tank may be cheaper and easier to construct; may be capable of supporting higher pressures; and/or makes better use of available storage space, due to the curved nature of the base.