This paper discusses the feasibility of coding an "undetectable" digital water mark on a standard 512*512 intensity image with an 8 bit gray scale. The watermark is capable of carrying such information as authentication or authorisation codes, or even a legend essential for image interpretation. This capability is envisaged to find application in image tagging, copyright enforcement, counterfeit protection, and controlled access to image data. Two methods of implementation are discussed. The first is based on bit plane manipulation of the LSB, which offers easy and rapid decoding commensurate with streaming type image processing. The second method utilises linear addition of the water mark to the image data, and is thus more difficult to decode, and therefore offers inherent security. f