A method of making first and second blade modules (24, 26) of a modular wind turbine blade, the first and second blade modules each having a spar cap section (44a, 46a) with a tapered end (52, 60), and the modules being configured for connection end-to-end by an adhesively bonded scarf joint between the tapered end portions of respective spar cap sections of the blade modules, and the method comprising: providing a blade mould having a first region, a second region and a joint region between the first and second regions; arranging a first outer skin layer of fibrous material in the first region of the mould and in the joint region of the mould to form an outer skin of the first blade module; arranging a first spar cap section having a tapered end portion on top of the first outer skin layer such that the tapered end portion is in the joint region of the mould and a remainder of the first spar cap section extends into the first region of the mould; arranging a first separating layer on top of the tapered end portion of the first spar cap section in the joint region of the mould; arranging a second outer skin layer of fibrous material in the second region of the mould to form an outer skin of the second blade module; arranging a second spar cap section having a tapered end portion in the mould such that its tapered end portion overlaps the tapered end portion of the first spar cap section in the joint region of the mould with the first separating layer sandwiched there between, and such that a remainder of the second spar cap section extends into the second region of the mould and is on top of the second outer skin layer; integrating the first outer skin layer with the first spar cap section to form a shell portion of the first blade module; integrating the second outer skin layer with the second spar cap section to form a shell portion of the second blade module; separating the shell portions of the first and second blade modules (24, 26); and selecting a thickness of the first separating layer to correspond to a required bond thickness of the scarf joint between the tapered end portions (52, 60) of the first and second spar cap sections (44a, 46a) when the first and second modules are connected together to form the modular blade. providing a blade mould having a first region, a second region and a joint region between the first and second regions; arranging a first outer skin layer of fibrous material in the first region of the mould and in the joint region of the mould to form an outer skin of the first blade module; arranging a first spar cap section having a tapered end portion on top of the first outer skin layer such that the tapered end portion is in the joint region of the mould and a remainder of the first spar cap section extends into the first region of the mould; arranging a first separating layer on top of the tapered end portion of the first spar cap section in the joint region of the mould; arranging a second outer skin layer of fibrous material in the second region of the mould to form an outer skin of the second blade module; arranging a second spar cap section having a tapered end portion in the mould such that its tapered end portion overlaps the tapered end portion of the first spar cap section in the joint region of the mould with the first separating layer sandwiched there between, and such that a remainder of the second spar cap section extends into the second region of the mould and is on top of the second outer skin layer; integrating the first outer skin layer with the first spar cap section to form a shell portion of the first blade module; integrating the second outer skin layer with the second spar cap section to form a shell portion of the second blade module; separating the shell portions of the first and second blade modules (24, 26); and selecting a thickness of the first separating layer to correspond to a required bond thickness of the scarf joint between the tapered end portions (52, 60) of the first and second spar cap sections (44a, 46a) when the first and second modules are connected together to form the modular blade.