A method of joining a functional component (30; 54; 90; 110) to a thermosetting polymer composite component (50; 92; 112), the method including the steps of: locating a thermoplastic surface of said composite component (50; 92; 112) in intimate contact with a thermoplastic surface (32; 52) of said functional component (30; 54; 90; 110); applying a high-frequency oscillating relative displacement between said thermosetting polymer composite component (50; 92; 112) and said functional component (30; 54; 90; 110), such that at least a portion of the respective thermoplastic surfaces of the components at least partly melt and fuse; halting the high-frequency oscillating relative displacement and holding said thermosetting polymer composite component and said functional component (30; 54; 90; 110) together in a fixed relationship, such that the molten thermoplastic or thermoplastics resolidifies and the components are welded together; characterized in that the thermosetting composite component (50; 92; 112) comprises a thermosetting polymer composite material, with a thermoplastic polymer layer on at least the part of the surface to be joined, the functional component (30; 54; 90; 110) being selected to have a second thermoplastic polymer at least on the surface (32; 52) to be joined, the second thermoplastic polymer being selected such that it can form high strength joints with the thermoplastic surface of said thermosetting polymer composite component (50; 92; 112) in high-frequency oscillating relative displacement welding operations, the method further comprising the steps of: forming the thermoplastic layer on the thermosetting polymer composite component (50; 92; 112) by selecting uncured thermosetting polymer composite resin constituents and a first thermoplastic polymer, that have Hansen solubility parameters indicative of the ability of the thermoplastic polymer and uncured thermosetting polymer resin constituents to interpenetrate; locating said thermoplastic polymer in contact with the uncured thermosetting polymer composite component (50; 92; 112), so that at least a part of one surface of the composite component when cured consists of said thermoplastic polymer; heating the thermoplastic polymer and uncured thermosetting polymer composite component (50; 92; 112) to the curing temperature of the thermosetting polymer, the uncured thermosetting polymer resin constituents and the thermoplastic polymer being able to at least partly interpenetrate before the thermosetting polymer cures; and cooling the thermoplastic polymer and cured thermosetting polymer component such that the thermoplastic polymer is very strongly bonded to the cured thermosetting polymer component (50; 92; 112). locating a thermoplastic surface of said composite component (50; 92; 112) in intimate contact with a thermoplastic surface (32; 52) of said functional component (30; 54; 90; 110); applying a high-frequency oscillating relative displacement between said thermosetting polymer composite component (50; 92; 112) and said functional component (30; 54; 90; 110), such that at least a portion of the respective thermoplastic surfaces of the components at least partly melt and fuse; halting the high-frequency oscillating relative displacement and holding said thermosetting polymer composite component and said functional component (30; 54; 90; 110) together in a fixed relationship, such that the molten thermoplastic or thermoplastics resolidifies and the components are welded together; forming the thermoplastic layer on the thermosetting polymer composite component (50; 92; 112) by selecting uncured thermosetting polymer composite resin constituents and a first thermoplastic polymer, that have Hansen solubility parameters indicative of the ability of the thermoplastic polymer and uncured thermosetting polymer resin constituents to interpenetrate; locating said thermoplastic polymer in contact with the uncured thermosetting polymer composite component (50; 92; 112), so that at least a part of one surface of the composite component when cured consists of said thermoplastic polymer; heating the thermoplastic polymer and uncured thermosetting polymer composite component (50; 92; 112) to the curing temperature of the thermosetting polymer, the uncured thermosetting polymer resin constituents and the thermoplastic polymer being able to at least partly interpenetrate before the thermosetting polymer cures; and cooling the thermoplastic polymer and cured thermosetting polymer component such that the thermoplastic polymer is very strongly bonded to the cured thermosetting polymer component (50; 92; 112).