A method of balancing a set of modular blade sections (310, 410, 510, 320, 420, 520) for a set of blades (30, 40, 50) of a rotor of a wind turbine, the method comprising the steps of: providing the set of modular blade sections, the set comprising a plurality of first blade sections (310, 410, 510) and a plurality of second blade sections (320, 420, 520); determining a mass moment of each first blade section (310, 410, 510) and of each second blade section (320, 420, 520), the mass moment being determined about an axis equivalent to an axis of rotation of a hub of the turbine when the blade section is assembled onto the hub; and defining a plurality of blade section groups, each comprising a first blade section (310, 410, 510) of the set and at least one second blade section (320, 420, 520) of the set, each blade section group having a blade section group mass moment (MMu) about the axis, the blade section group mass moment (MMu) being the sum of the mass moments of the blade sections within the group, the first blade section and the at least one second blade section in each group being selected so as to: minimise a sum of differences between: the group mass moment of the blade section group having the highest group mass moment of the plurality of blade section groups; and the group mass moment of each of the remaining blade section groups; or minimise a difference in mass moment between: the group mass moment of the blade section group or groups having the highest group mass moment of the plurality of blade section groups; and the group mass moment of the blade section group or groups having the lowest group mass moment of the plurality of blade section groups. providing the set of modular blade sections, the set comprising a plurality of first blade sections (310, 410, 510) and a plurality of second blade sections (320, 420, 520); determining a mass moment of each first blade section (310, 410, 510) and of each second blade section (320, 420, 520), the mass moment being determined about an axis equivalent to an axis of rotation of a hub of the turbine when the blade section is assembled onto the hub; and defining a plurality of blade section groups, each comprising a first blade section (310, 410, 510) of the set and at least one second blade section (320, 420, 520) of the set, each blade section group having a blade section group mass moment (MMu) about the axis, the blade section group mass moment (MMu) being the sum of the mass moments of the blade sections within the group, the first blade section and the at least one second blade section in each group being selected so as to: minimise a sum of differences between: the group mass moment of the blade section group having the highest group mass moment of the plurality of blade section groups; and the group mass moment of each of the remaining blade section groups; or minimise a difference in mass moment between: the group mass moment of the blade section group or groups having the highest group mass moment of the plurality of blade section groups; and the group mass moment of the blade section group or groups having the lowest group mass moment of the plurality of blade section groups. minimise a sum of differences between: the group mass moment of the blade section group having the highest group mass moment of the plurality of blade section groups; and the group mass moment of each of the remaining blade section groups; or the group mass moment of the blade section group having the highest group mass moment of the plurality of blade section groups; and the group mass moment of each of the remaining blade section groups; or minimise a difference in mass moment between: the group mass moment of the blade section group or groups having the highest group mass moment of the plurality of blade section groups; and the group mass moment of the blade section group or groups having the lowest group mass moment of the plurality of blade section groups. the group mass moment of the blade section group or groups having the highest group mass moment of the plurality of blade section groups; and the group mass moment of the blade section group or groups having the lowest group mass moment of the plurality of blade section groups.