Abstract: A hearing instrument and external fitting device for adjusting, or “trimming”, the circuit characteristics of a hearing instrument. The external fitting device includes a set of external trimmer elements which are connected into the hearing instrument internal circuitry, where the values of the trimmer elements are periodically sampled by the instrument's circuitry when the fitting device is connected to the instrument, the values of the trimmer elements being stored in an internal memory to control the characteristics of the hearing instrument. When a satisfactory set of adjustment values are achieved, the fitting device is disconnected from the hearing instrument until required for further adjustments. A fitting card is associated with each instrument and identifies the instrument or user of the instrument and may be fitted to the fitting device to record the settings of the trimmer elements.