A modular wind turbine blade is described. The blade has at least a first blade module and a second blade module connected to end to end. An interface is defined between the respective modules. The blade further comprises at least one longitudinally extending spar structure that forms part of the outer shells of the first and second blade modules when the modules are connected. The or each spar structure is substantially continuous and extends across the interface between the first and second blade modules when the modules are connected. The or each spar structure is integrated with the first blade module and the spar structure is folded or coiled to facilitate transportation of the module. The second blade module includes channels defined in the outer shell for receiving the or each spar structure when the modules are connected. In this way the spar structures also form part of the outer shell of the second module. A method of assembling the modular blade and a method of installing a wind turbine having a modular blade are also described.