Although a number of studies have shown impaired tone and speech sound perception in aphasia, the contribution of these impairments to deficits in auditory language comprehension remains unclear. This study investigates electrophysiological aspects of tone and speech sound processing in aphasia. Passive and active tone and speech sound discrimination were investigated using event-related brain potentials (ERPs) in one longitudinal and three horizontal studies. Which processing stages are disturbed in aphasia and whether there are differences in tonal vs. speech sound processing and in active vs. passive processing, were main research questions of this present study. Of special interest were possible changes of topographical distribution and activation changes over time. Also the possible use of ERPs in aphasia rehabilitation was studied. The study’s results suggest disturbed speech sound processing in several processing stages. Reduced brain activity already 100 ms after stimulus onset was found to correlate with aphasia severity. The results further indicate that disturbances at this processing stage are specific for aphasia and not caused by general brain damage effects only. While also tone perception was changed at this early processing level, additional speech-specific processing impairments seem to be present. Later processing stages were changed in complex patterns which might be explained by possible subgroups of aphasic patients. Several results suggest compensatory activation of the hemisphere contralateral to the brain damage. At least parts of this compensational activation seem to be specific for speech sounds. Furthermore, changes of activation during recovery from aphasia and inter-individual differences were observed. ERPs might play a role in assessing these differences in clinical settings and might be of value in prognostic considerations.