Dutch is a so-called verb-second language, i.e. the finite verb typically occurs in second position with one constituent in initial position. The element in initial position is normally the subject. However, if it is an object or an adverb, the subject occurs after the finite verb as, for example, in Gisteren las Karel een boek (Yesterday read Charles a book), Dit boek las Karel gisteren (This book read Charles yesterday). This is what is called Inversion. Both in child first- and adult second-language acquisition inversion is acquired as a function of topicalization. As part of a topicalization device auxiliary verbs (with past participles) play an important role. First, non-modal auxiliaries (AUX) such as heb (have) or heeft (has), then also modal auxiliaries such as kan, mag, moet, wil (can, may, have-to, want) etc. For example: Dat heb ik nooit gezegd (That have I never said), Die wil ik hebben (That want I have). The functioning of the structure Topic + AUX + subject pronoun as a topicalization device explains, why auxiliaries, subject pronouns and inversion are acquired simultaneously and why the relevant processes of L1 and L2 acquisition are the same.