The Nucleus Multi-channel Implantable Hearing Prosthesis was developed in conjunction with the University of Melbourne. A 22 channel flexible electrode array is inserted into the cochlea through a round window opening. A cable leads from the electrode array to a demountable connector mounted on the Receiver-Stimulator unit (RSU). A custom CMOS integrated circuit is mounted inside the hermetically sealed titanium and ceramic RSU which is encased in silicone rubber and measures 34.5 mm diameter and 10.5 mm thick. Biphasic current pulse stimuli can be delivered to any selected electrode pairs at rates in excess of 1 kHz. A pocket-sized speech processor extracts spectral information from the incoming acoustic signal, and encodes stimulus information onto the single RF power/data signal transmitted to the RSU through the skin. The Speech Processor includes electronic memory circuits which are programmed with the information mapping acoustic parameters to stimulation parameters for each electrode for each patient. This MAP is established as a result of psychophysical experiments performed by the audiologist using the Diagnostic and Programming System based on a customized microcomputer.