Abstract In Norwegian and Swedish, the auxiliary ha 'have' can sometimes be omitted. Earlier analyses have treated this as a unitary phenomenon. However, while ha can only be left out after modal verbs in Norwegian, ha-omission has a wider distribution in Swedish. This indicates that the optionality of ha depends on more than one factor. In constructions with modal verbs, the head that is spelled out as ha can be present or absent in both languages. That is, the optionality of ha is connected to the optionality of generating the relevant head. In Swedish, there is the additional possibility of generating ha without spelling it out. More precisely, ha need not be spelled out if ha shares its features with some element that is overtly realized. Still, there is much variation between speakers of Swedish concerning the distribution of optional ha. It appears that a number of grammatical factors contribute to the optionality of a phonologically realized ha.