A method for forming a high aspect ratio microstructure (20) comprises: a) forming on a conductive surface (4) of a carrier substrate (3) one or more dielectric structures (6) to create a mandrel (7); b) forming masking material (8) by electrodepositing on exposed areas of the conductive surface of the mandrel one or more metal structures; c) forming a composite structure (18) by taking a photopolymer structure comprising a substrate (12) having thereon an electromagnetic radiation-sensitive photopolymer of either a positive tone or a negative tone (14), the photopolymer having a thickness substantially equal to the desired height of the microstructure to be formed, and adhering the photopolymer to the masking material (8), wherein the masking material is opaque to the electromagnetic radiation; d) removing the carrier (3); e) exposing the photopolymer (14) to electromagnetic radiation so as to irradiate regions of the photopolymer corresponding to the one or more dielectric structures (6) and substantially not to irradiate regions of the photopolymer corresponding to the one or more metal structures (8); and f) developing to selectively remove those portions of the photopolymer exposed to the radiation if the photopolymer has a positive tone; or to selectively remove the unexposed portions of the photopolymer if the photopolymer has a negative tone, thereby forming a high aspect ratio microstructure (20). The invention also provides a composite structure (18) for use in the method.