Vocal communication plays an important role in the social life of baboons, yet so far the literature has only touched on their vocalizations. The structure of different calls has not been described in any detail, but modern sonographic techniques now enable us to analyse the various features of baboon sounds. The subject is the Windsor Safari Park's troop of Hamadryas baboons Papio hamadryas. The troop currently numbers 48 individuals, organized as four harem families with male followers, and kinship relations are known. Eight broad categories of calls have so far been identified. Their acoustic features correspond to expectations for vocal communication in an open habitat. There are only subtle structural changes from one category to the next. There is variation in the detailed structure of vocalizations within one category and certain vocalizations go together with specific facial expressions and body postures. Conditions permit close study of each individual's ranking in the troop; vocalizations can therefore be related to rank as well as specific behavioural context. For example, there is now evidence that explosive barks are used to communicate where visual contact cannot be established.