What is now known as partial deafness treatment (PDT) began as a great challenge: insert a cochlear implant and preserve residual preoperative hearing. PDT addresses the situation where patients have low frequency hearing which is not badly affected, or is even fully functional, and it is necessary to complement it electrically in the medium and high frequency ranges. The first surgery in which existing normal hearing was electrically complemented, performed 10 years ago, has opened up a new direction in the treatment of other (more frequent) partial hearing impairments. The long term observation of more than 1200 ears has allowed us to refine a new strategy of PDT. The first surgery to electrically complement the existing hearing of an adult was performed in 2002, and led in 2004 to the same treatment in a child. It was preceded with 3 years of research on how to enhance the low-frequency residual hearing of patients by amplifying it with conventional hearing aids while supplying high frequencies with an electrically stimulated implant. Results of these studies, presented in 2000 at international conferences, demonstrated the potential for widening the first, cautious indications for the use of cochlear implants to situations where there are various levels of residual low-frequency hearing. Experience with 1200 cases of partial deafness surgery have proven its general validity and have refined the surgical guidelines. The surgical strategy is based on 6 main steps involving an approach to scala tympani through the round window, and use of electrodes with an active length of 20 to 28 mm.