A security device comprising: an array of elongate focusing structures, the elongate axes of which are aligned along a first direction (y), the elongate focusing structures being arranged parallel to one another periodically along a second direction which is orthogonal to the first direction (x), each elongate focusing structure having an optical footprint of which different elongate strips will be directed to the viewer in dependence on the viewing angle, the centre line of each optical footprint being parallel with the first direction; and an array of image elements (5a, 5b) overlapping the array of elongate focusing structures, the array of image elements representing elongate image slices of at least two respective images (I A, I B), each image slice comprising one or more image elements, and at least one image slice of each respective image being located at least partially in the optical footprint of each elongate focusing structure; characterised in that the array of image elements is configured such that the pitch between the elongate image slices of each respective image in the second direction varies across the array in the first and/or second direction(s); whereby, at any one viewing angle, in a first region of the device the elongate focussing structures direct portions of first image slices corresponding to a first image (I A) to the viewer such that the first image is displayed across the first region of the device, and simultaneously, in a second region (R 2) of the device which is laterally offset from the first region in the first and/or second direction(s), the elongate focussing structures direct portions of second image slices corresponding to a second image (I B) to the viewer such that the second image is displayed across the second region of the device, the positions of the first and second regions relative to the security device depending on the viewing angle. an array of elongate focusing structures, the elongate axes of which are aligned along a first direction (y), the elongate focusing structures being arranged parallel to one another periodically along a second direction which is orthogonal to the first direction (x), each elongate focusing structure having an optical footprint of which different elongate strips will be directed to the viewer in dependence on the viewing angle, the centre line of each optical footprint being parallel with the first direction; and an array of image elements (5a, 5b) overlapping the array of elongate focusing structures, the array of image elements representing elongate image slices of at least two respective images (I A, I B), each image slice comprising one or more image elements, and at least one image slice of each respective image being located at least partially in the optical footprint of each elongate focusing structure; characterised in that the array of image elements is configured such that the pitch between the elongate image slices of each respective image in the second direction varies across the array in the first and/or second direction(s); whereby, at any one viewing angle, in a first region of the device the elongate focussing structures direct portions of first image slices corresponding to a first image (I A) to the viewer such that the first image is displayed across the first region of the device, and simultaneously, in a second region (R 2) of the device which is laterally offset from the first region in the first and/or second direction(s), the elongate focussing structures direct portions of second image slices corresponding to a second image (I B) to the viewer such that the second image is displayed across the second region of the device, the positions of the first and second regions relative to the security device depending on the viewing angle.