A data communication system comprising:- a data communication device (1, 101) for attachment to an object, the device comprising:- a flexible substrate; a transmitter adapted to transmit data in the form of electromagnetic radiation, wherein the data that is transmitted comprises descriptive data and content data, the descriptive data defining the content; a programmable memory; a controller for controlling the transmission of the data; a programming means for programming the device; and, a power source having:- i) a power storage; and, ii) a power extraction means capable of extracting power from the environment in which the data communication device is located when in use; the data communication system comprising further a portable reader for receiving electromagnetic radiation from the device; wherein the reader comprises a perceptive filter adapted to distinguish radiation received from the device from radiation received from other sources, the perceptive filter being adapted to use the descriptive data to determine whether the content data is relevant to the reader and, if it is relevant, to process the content data, and, if the descriptive data provides insufficient information, to examine the content data to decide if the content is relevant to the application of the reader. a flexible substrate; a transmitter adapted to transmit data in the form of electromagnetic radiation, wherein the data that is transmitted comprises descriptive data and content data, the descriptive data defining the content; a programmable memory; a controller for controlling the transmission of the data; a programming means for programming the device; and, a power source having:- i) a power storage; and, ii) a power extraction means capable of extracting power from the environment in which the data communication device is located when in use; the data communication system comprising further a portable reader for receiving electromagnetic radiation from the device; wherein the reader comprises a perceptive filter adapted to distinguish radiation received from the device from radiation received from other sources, the perceptive filter being adapted to use the descriptive data to determine whether the content data is relevant to the reader and, if it is relevant, to process the content data, and, if the descriptive data provides insufficient information, to examine the content data to decide if the content is relevant to the application of the reader. i) a power storage; and, ii) a power extraction means capable of extracting power from the environment in which the data communication device is located when in use; the data communication system comprising further a portable reader for receiving electromagnetic radiation from the device; wherein the reader comprises a perceptive filter adapted to distinguish radiation received from the device from radiation received from other sources, the perceptive filter being adapted to use the descriptive data to determine whether the content data is relevant to the reader and, if it is relevant, to process the content data, and, if the descriptive data provides insufficient information, to examine the content data to decide if the content is relevant to the application of the reader.