A security device has a lenticular device comprising an array of lenticular focusing elements (2); a corresponding array of sets of image strips (1-4), the sets of image strips having substantially the same periodicity as, or an integral multiple of the periodicity of, the array of lenticular focusing elements. The image strips (1-4) are formed and the focusing elements (2) located relative to the image strips such that at each of a plurality of viewing angles (A-D), a respective one image strip from each set is viewed in response to incident light falling on a respective lenticular focusing element. The image strips (1-4) are constructed such that the device presents a cyclically repeating sequence of images as the device is viewed at successive viewing angles, the image sequence comprising a change in form of an image between a first form and a second form and then a reversal of the sequence back to the first form, and the combined image sequences presenting a contiguous variation in the image form.