Abstract: A method of designing a structure, such as an airfoil section. A first set of candidate designs is generated in a first optimisation process, each candidate design comprising a set of M design variables associated with an M-dimensional design space. A subset of the first set of candidate designs is selected. The selected subset of candidate designs is analysed by proper orthogonal decomposition or principal component analysis to generate an N-dimensional design space defined by N design variables, N being less than M. A second set of one or more candidate designs is then generated in a second optimisation process, each candidate design comprising a set of N design variables associated with the N-dimensional design space.