Abstract: Apparatus for monitoring changes in a physical quantity with time comprises a reader unit and at least one sensor unit containing electronic components arranged to measure said quantity at predetermined time intervals and to store measurement data with corresponding time data. Each sensor unit also contains communication means for transferring said measurement data and said time data from the sensor unit to the reader unit, and the sensor unit also contains an inductive coupling means to receive energy from the reader unit to power the communication means. Each sensor unit is encapsulated with a food grade plastic and is typically the size of a credit card, thereby providing a small and relatively inexpensive temperature monitoring device for use in the transport and storage of foodstuffs.