Disclosed is a method of authenticating that a test polymer is an authenticatable polymer, wherein the authenticatable polymer has an authentication signal and comprises a substrate polymer and an optically variable tag, the optically variable tag having a fluorescence emission whose wavelength and/or intensity change over time, the method comprising subjecting the test polymer to a stimulus sufficient to cause fluorescence of the optically variable tag, determining a test signal from the fluorescence of the test polymer, and authenticating that the test polymer is an authenticatable polymer if the test signal is the same as the authentication signal of the authenticatable polymer.