Speech is only useful when it is transmitted from a speaker to a listener. Very often this is done in a telecommunication setting. If we want to transmit speech often there are restrictions that include limited audio and/or network bandwidth. Therefore, one issue is to use the available resources - while maximizing the speech quality - by finding efficient speech coding and error concealment strategies. In a real-world-environment the speech signal cannot be picked up with perfect quality, e.g. when the speaker is driving a car. This results in efforts to enhance the speech quality for the listener. Tasks include noise suppression, where based on the statistics of the background noise, we try to remove those unwanted signal components from the noisy speech signal. Further, in a hands-free communication scenario, echo cancellation is used to subtract the signal from the loudspeaker that is picked up again by the microphone, which would otherwise be heard by the far-end speaker as echo of his voice. For low audio-bandwidth signals, artificial bandwidth extension can improve the audio signal quality considerably. Any enhancement effort can also deteriorate the desired speech sound, so minimizing this effect is an important task. Finally, the enhancement of speech as produced by humans suffering from voice pathologies is an important application area.