A modular tank comprises one or more wall modules coupled together and one or more liner bags supported by the one or more wall modules for the containment of liquid, in particular water. The wall modules can be planar or curved and the edges of the wall modules are coupled together via joining members. The modular tank comprises one or more layers of the wall modules. One or more of the wall modules comprise a hole and the one or more liner bags, which can be coupled together, comprise one or more parts that protrude through the holes in the wall modules to support the liner bags.