In Swedish it is possible to omit the temporal auxiliary ha ‘have’, in certain contexts. Infinitival ha can be omitted in modal verb phrases in main clauses (1) whereas finite ha can be omitted in subordinate clauses (2). (1) Pjäsen borde (ha) börjat nu. The play should (have) begun now ‘The play should have begun now’ (2) Hon berättade att hon (hade) sett honom tidigare. She told that she (had) seen him earlier ‘She told me that she had seen him earlier’ The ha-omission phenomenon emerged by the end of the Early Modern Swedish period, i.e. in the end of the 17th century. Traditionally, two explanatory circumstances have been suggested. One is that ha-omission is a syntactic loan from German and the other that it is a written language phenomenon (cf. Johannisson 1945, Platzack 1983, Malmgren 1985). A theoretically grounded description of the origin and evolution of ha-omission is yet to be made. Hitherto only a few synchronic studies within a generative framework have been presented (e.g. Julien 2002). In the last decade diachronic construction grammar has been demonstrated as a useful tool to expose the mechanisms at work within this kind of language change (Traugott & Trousdale 2013, Barðdal et al. in press, Hilpert 2013, Bergs & Diewald 2008). I will present a diachronic study of ha-omission in Swedish and discuss some possible explanations to why the temporal auxiliary is omitted. Could the phenomenon have to do with grammaticalization concerning the auxiliary and/or the participle? Or could it be a matter of constructionalization of the larger constructions in which they occur?