Apparatus for monitoring and or treating a substance or body. The apparatus comprises a sensor 1, at least a portion of which 6 is arranged in use to come into contact with a body, tissue, bodily fluid or other substance, and a computer device 3. Both the sensor 1 and the computer device 3 have radio frequency circuitry 12,16 for facilitating the transmission of data between them via a wireless communication link, the computer device 3 having means 15,17 for selecting wireless link transmission properties based upon user location, and for signalling these properties to the sensor 1.