A security element 2, 4 for embedding in or application to a security document in such a way that it is visually recognizable from both sides of the security document 1, is structured in a multilayer fashion and includes two interference elements I1, I2 with color shift effect, a metallic reflection layer R located in between as well as, optionally, diffraction structures 8. Depending on the disposition of the layers I1, R, I2 and the optionally present diffraction structures 8 on a transparent substrate S the color shift effect and/or the diffractive effects are perceptible from one or from both sides of the security element 2, 4. The security element is particularly suitable as a two-sided window thread 4 and as a label or transfer element 2 above a hole 3 in the security document 1.