This paper examines variation in the singing behaviour of male and female gibbons in an attempt to construct a consistent framework to describe the function of both solo and duet song bouts. Functional hypotheses are described and tested with the available data. The song bouts of mated females are shown to be strongly associated with the pressure of territorial defence. Conversely, the solo song bouts of mated males show no relationship with territorial behaviour, but are associated with the (estimated) density of floating unmated males in the population. In accordance with the hypotheses under test, these results indicate that mated females sing to defend their territories, whilst mated males sing to repel males in defence of their mate and offspring. Contrary to previous hypotheses, the production of coordinated duets by male and female pairs is found to be unrelated to pairbonding. However, duetting is shown to be associated with encounters and aggression at the territorial boundary, in support of the claim that duetting advertises pairbond presence in order to intimidate neighbours and reduce the costs of territorial conflicts. The implications of these analyses for territorial and mating strategies in gibbons are discussed.