An increasing number of telephone services are offered in a fully automatic way with the help of speech technology. The underlying systems, called spoken dialogue systems (SDSs), possess speech recognition, speech understanding, dialogue management, and speech generation capabilities, and enable a more-or-less natural spoken interaction with the human user. Nevertheless, the principles underlying this type of interaction are different from the ones which govern telephone conversations between humans, because of the limitations of the machine interaction partner. Users are normally able to cope with the limitations and to reach the goal of the interaction, provided that both interlocutors behave in a cooperative way
