A method is used to determine focus of a lithographic apparatus used in a lithographic process on a substrate. The lithographic process is used to form at least two periodic structures on the substrate. Each structure has at least one feature that has an asymmetry between opposing side wall angles that varies as a different function of the focus of the lithographic apparatus on the substrate. A spectrum produced by directing a beam of radiation onto the at least two periodic structures is measured and ratios of the asymmetries are determined. The ratios and a relationship between the focus and the side wall asymmetry for each structure is used to determine the focus of the lithographic apparatus on the substrate.