Although it is one of the most conspicuous features of dog behaviour, barking has received little attention from ethologists or from an applied perspective. In this review, an ethological look is taken at the communicative aspect of dog barking. Emerging new research has indicated that in the repertoire of dog vocalisations barking has unique features in showing wide ranges of acoustic parameters, such as frequency, tonality and rhythmicity. Barking has been shown to be context dependent, and provides information for humans about the inner state of the dog although there are few indications that barking is used for intra-species communication. It is assumed that dog barking emerged through selective processes in which human preferences for certain acoustic aspects of the vocalisation may have been paramount. A more experiment-oriented approach is required for the study of dog vocalisation that could shed light on the possible communicative function of these acoustic signals. Copyright (c) 2009 Elsevier Ltd. All rights reserved.