Abstract: A method of producing a composite shell structure in the form of a wind turbine blade shell part having a reinforced fiber material embedded in a cured resin includes: providing a mold part having a contour defining the outer surface of the composite shell structure; providing a preform forming part shaped complementary to the mold part molding surface; forming a preform of dry fiber material on the surface of the preform forming part; arranging the mold part and the preform forming part in an assembled position with the mold part molding surface facing the preform forming surface; releasing the preform from the preform forming surface so that it is received on the mold part molding surface; removing the preform forming part from the mold part; forming a mold cavity by means of a second mold part arranged on the preform; providing resin in the mold cavity; and curing the resin.