In this article a novel method for structural optimization of laminated composite shell structures such as wind turbine blades is presented. The outer shape of a wind turbine blade is typically determined by aerodynamic considerations and therefore not subject to change. Furthermore, the thicknesses of the shell structures are also considered fixed. The design objective is chosen to be a global quantity such as maximum stiffness or lowest eigenfrequency with a constraint on the total mass, such that the cost of material can be considered. The design optimization method is based on ideas from multiphase topology optimization where the material stiffness (or density) is computed as a weighted sum of candidate materials, and the method is easy to implement in existing finite element codes. The potential of the method to solve the combinatorial problem of proper choice of material, stacking sequence and fibre orientation simultaneously for maximum stiffness or lowest eigenfrequency design is illustrated on both small test examples and a real-life main spar from a wind turbine blade. Copyright © 2004 John Wiley & Sons, Ltd.