A method of measuring focus performance of a lithographic apparatus, the method comprising: (a) using the lithographic apparatus to print at least one focus metrology pattern on a substrate, the printed focus metrology pattern comprising an array of features that is periodic in at least one direction; (b) measuring a property of the printed focus metrology pattern (1020, 1030); and (c) deriving a measurement of focus performance from the measurement of said property (1040, 1050), wherein the focus metrology pattern is defined by a patterning device and the printing in step (a) is performed by illuminating said patterning device with patterning radiation incident at an oblique angle (θ), and wherein the focus metrology pattern comprises a periodic array of groups of first features, each group comprising one or more first features and wherein a spacing between adjacent groups of first features within the focus metrology pattern is much greater than the dimension of each first feature in the direction of periodicity; and wherein each of said one or more first features is within the printing resolution of the lithographic apparatus and has a dimension less than 50 nanometers in the direction of periodicity when printed on the substrate, characterized in that the measured property of the printed focus metrology pattern arises from a focus dependent displacement of said one or more first features in the direction of periodicity. (a) using the lithographic apparatus to print at least one focus metrology pattern on a substrate, the printed focus metrology pattern comprising an array of features that is periodic in at least one direction; (b) measuring a property of the printed focus metrology pattern (1020, 1030); and (c) deriving a measurement of focus performance from the measurement of said property (1040, 1050), wherein the focus metrology pattern is defined by a patterning device and the printing in step (a) is performed by illuminating said patterning device with patterning radiation incident at an oblique angle (θ), and wherein the focus metrology pattern comprises a periodic array of groups of first features, each group comprising one or more first features and wherein a spacing between adjacent groups of first features within the focus metrology pattern is much greater than the dimension of each first feature in the direction of periodicity; and wherein each of said one or more first features is within the printing resolution of the lithographic apparatus and has a dimension less than 50 nanometers in the direction of periodicity when printed on the substrate, characterized in that the measured property of the printed focus metrology pattern arises from a focus dependent displacement of said one or more first features in the direction of periodicity. wherein the focus metrology pattern is defined by a patterning device and the printing in step (a) is performed by illuminating said patterning device with patterning radiation incident at an oblique angle (θ), and wherein the focus metrology pattern comprises a periodic array of groups of first features, each group comprising one or more first features and wherein a spacing between adjacent groups of first features within the focus metrology pattern is much greater than the dimension of each first feature in the direction of periodicity; and wherein each of said one or more first features is within the printing resolution of the lithographic apparatus and has a dimension less than 50 nanometers in the direction of periodicity when printed on the substrate, characterized in that the measured property of the printed focus metrology pattern arises from a focus dependent displacement of said one or more first features in the direction of periodicity.