A security device comprising: an array of elongate focusing structures, the elongate axes of which are aligned along a first direction, the elongate focusing structures being arranged parallel to one another periodically along a second direction which is orthogonal to the first direction, 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 overlapping the array of elongate focusing structures, the array of image elements representing elongate image slices of at least two respective images, each image slice comprising one or more image elements, and at least one image slice of each respective image being located in the optical footprint of each elongate focusing structure; wherein the array of image elements is configured such that the distance in the second direction of each image slice from the centre line of an optical footprint in which the image slice is located changes along the first direction; whereby, at at least some viewing angles, the elongate strip of the optical footprint of each elongate focusing structure which is directed to the viewer includes a portion of a first image slice corresponding to a first image and a portion of a second image slice corresponding to a second image, such that the first image is displayed by a first region of the security device and the second image is displayed by a second region of the security device which is laterally offset from the first region in the first direction, the positions of the first and second regions along the first direction depending on the viewing angle. an array of elongate focusing structures, the elongate axes of which are aligned along a first direction, the elongate focusing structures being arranged parallel to one another periodically along a second direction which is orthogonal to the first direction, 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 overlapping the array of elongate focusing structures, the array of image elements representing elongate image slices of at least two respective images, each image slice comprising one or more image elements, and at least one image slice of each respective image being located in the optical footprint of each elongate focusing structure; wherein the array of image elements is configured such that the distance in the second direction of each image slice from the centre line of an optical footprint in which the image slice is located changes along the first direction; whereby, at at least some viewing angles, the elongate strip of the optical footprint of each elongate focusing structure which is directed to the viewer includes a portion of a first image slice corresponding to a first image and a portion of a second image slice corresponding to a second image, such that the first image is displayed by a first region of the security device and the second image is displayed by a second region of the security device which is laterally offset from the first region in the first direction, the positions of the first and second regions along the first direction depending on the viewing angle.