Rehabilitative audiology is a quiet young, interdisciplinary field of, research, which is concerned with the diagnosis of hearing impairment, and its rehabilitation with hearing instruments. Due to the introduction, of digital technology in modern hearing instruments, major advancements, have been made in this field in recent years. However, these advances, primarily are concerned with technical issues, such as, e.g., advanced, signal processing techniques for noise reduction, suppression of, feedback and increase of listening as well as handling comfort. The, techniques for restoration of a patients individual deficiencies, on the, other hand, are still limited by our limited knowledge about the effect, of hearing impairment on the individual listener. Hence, a major, research effort is needed for a better understanding of the individual, hearing impairment in order to derive better processing techniques for, the compensation of the individual listeners' hearing handicap. This, thesis is directed towards this aim. For the diagnosis and treatment of, hearing disorders, hearing abilities of hearing-impaired listeners are, classically characterized by the audiogram, i.e., the loss of hearing, sensitivity as a function of frequency. However, it is widely known that, people suffering from cochlear hearing loss exhibit alterations in, several auditory functions, such as loudness perception, intensity, discrimination, frequency selectivity, temporal resolution and speech, perception [for an overview see Moore, 1995]. Hence, supra-threshold, hearing might be as important for the treatment of the individual, sensorineural hearing-impaired listener as the sensitivity loss, measured by the audiogram. The probably most important quantity for, characterizing supra-threshold perception has been described with the, term loudness, i.e., the subjective sensation corresponding to the, physical level of the acoustical stimulus which is also influenced by a, number of other issues (e.g., temporal and spectral structure of the, signal). Loudness and loudness perception is a major issue in, psychophysics which can be quantified for normal listeners in terms of, loudness models [e.g., Fletcher and Munson, 1933; Moore and Glasberg,, 1996; Stevens, 1956; Zwicker, 1958; Zwislocki, 1965]. Thereby, loudness, models aim at predicting perceived loudness from the physical properties, of the sound by considering psychophysical principles (such as the, pioneering assumptions of Fechner , Weber  and Stevens, ) as well as physiological and psychoacoustical findings. The, finding of an altered loudness perception in sensorineural, hearing-impaired listeners, i.e., the so called recruitment phenomenon, [Brunt, 1994; Fowler, 1936; Steinberg and Gardner, 1937] (the phenomenon, that once the level of a sound is increased above threshold, the, loudness increases more rapidly than in normal-hearing listeners), has, also been described and discussed in the literature for many years., Based on the classical model considerations by Fletcher [Fletcher and, Munson, 1933; Fletcher and Steinberg, 1924] and Stevens [Stevens, 1953,, 1956], Zwicker [Zwicker, 1958; Zwicker and Fastl, 1990] developed a, loudness model which has been the basis for a variety of modifications, and improvements both to predict recent data and to predict the effect, of hearing impairment [Chalupper, 2000; Florentine and Zwicker, 1979;, Launer, 1995; Launer et al., 1996, 1997; Marzinzik et al., 1996b; Moore, and Glasberg, 1996, 1997; Moore et al., 1996, 1997, 1999b, 2000, 2001;, Paulus and Zwicker, 1972]. Since any compensation for distorted loudness, perception in hearing-impaired listeners provided by a hearing aid, should be based on a solid understanding of the nature of loudness, perception, the development and evaluation of appropriate loudness, models is of major importance in rehabilitative audiology. This, motivated the work in the current thesis. The thesis is organized as, follows: It starts with a description of a field-test performed with a, prototype digital signal processing hearing aid, which aims at restoring, loudness perception in the individual hearing-impaired listeners in a, limited number of frequency channels with different algorithmic, approaches (mainly with respect to dynamic compression algorithms,, chapter 2). Since the limitations of the assessment of loudness, perception in this field-test and its compensation by the hearing aid, is obvious, the next chapter (chapter 3) is devoted towards appropriate, measurement methods for assessing loudness perception in normal- and, hearing-impaired listeners. Specifically the loudness matching task, (i.e., adjustment of the level of two different stimuli to yield the, same loudness impression) is compared to categorical loudness scaling, (i.e., direct numbering of the perceived loudness impression by the, subject with given categorical units). Although both methods are found, to be quite inaccurate in quantifying loudness summation for individual, listeners, the loudness scaling method has the advantage of a higher, applicability in the clinics. Hence, the reminder of the thesis is, devoted towards modeling loudness perception for stationary sounds in, normal- and hearing-impaired listeners based on data derived from the, method of loudness scaling. While chapter 4 gives a thorough review of, the literature on different versions of the loudness models based on the, approach by Zwicker, a new loudness model (The ''Oldenburg Loudness, Model'') is introduced in chapter 5, which has the advantage of better, predicting the individual data of hearing-impaired listener and taking, into consideration the most recent data on equal-loudness level, contours. Finally chapter 6 performs an evaluation of this model in, comparison with two other models known from the literature.