Difficult legal issues have arisen in recent cases concerning the separation of conjoined (Siamese) twins. If the inevitable result of separating conjoined twins is that one twin will die, does a surgeon performing the operation commit murder? In all Australian states and territories, it is murder to perform an action knowing that it will kill another human being, unless there are circumstances to ground one of a limited number of special defences. A defence called 'necessity' has most commonly been invoked in attempts to excuse causing the death of one conjoined twin in order to benefit the other. That defence, however, is notoriously uncertain in its application. In the event of a murder conviction, some jurisdictions make a sentence of life imprisonment mandatory; other jurisdictions give the judge discretion to impose a sentence up to life imprisonment. What would be an appropriate sentence in the event that a surgeon committed murder by separating conjoined twins?