For the treatment of hearing impairments, various types of hearing devices and surgical methods exist. However, these methods are not adequate for all types of hearing losses and patients' needs. Depending on the design, some of the inherent shortcomings could be reduced by implantable hearing systems. In implantable hearing systems, the output is generated by an implantable transducer, i.e. the equivalent of the loudspeaker of a conventional hearing aid. Two principles of implantable hearing systems which have been evaluated and developed are presented in this book. The first transducers CLT (i.e. contactless transducer) is intended for the rehabilitation of sensorineural hearing loss and couples to ossicles of the middle ear. It consists of a coil and magnet, both implantable in the middle ear in a minimally invasive manner. The second transducer DACS (i.e. Direct acoustical cochlear stimulation) is intended for the rehabilitation of severe combined perceptive and conductive hearing loss. It couples directly to the inner ear fluid by means of an amplified stapes prosthesis.