Abstract: An identifier comprising at least two machine readable components (62, 64) wherein at least one of the components (64) is capable of being changed from an original value to a modified value in response to a stimulus. In particular it provides for identifiers having codes such as bar codes or pictographic codes, with part of the codes being formed from a material which changes colour in response to a stimulus such as heat, atmosphere or flow of an electric current such that the value of the code as read by a machine may be changed to indicate that the identifier has been exposed to the stimulus. Also disclosed in an identifier formed by applying a light coloured material over a dark coloured surface such that gaps in the light coloured material form a machine readable code.