In previous papers (Vainikka & Young Scholten, 1994; in press a; and in press b), we proposed that in the acquisition of German, first language learners - like second language learners - gradually build up a syntactic structure. That is they posit only lexical projections at first, and then gradually posit the relevant functional projections. In this paper, we examine the role of triggers in second language acquisition. Specifically, we ask given that in second language, as in first language acquisition, syntactic projections gradually emerge, and given the notion that something becomes available in the input to trigger the reorganization of the L2 grammar, exactly what triggers second language development? We first describe the gradual building up of syntactic structures - or tree growth - in each of the stages of L2 acquisition (as described in Vainikka (1993/4)). We then consider what might constitute the relevant triggers of each of the stages of both L1 and L2 acquisitions (seen below). We find that the status of triggers in first and second language acquisition differs. We also observe that a number of learners in the ZISA studies (Clahsen & Muysken 1986) and in our LEXLEARN project in Dusseldorf appears to be fossilized. One might conclude that it is the different status of triggers for second language learners - rather than lack of access to Universal Grammar - that results in the lack of ultimate attainment of native competence. Since much of syntax is encoded in grammatical elements realized as affxes, difficulty in analyzing such affixes could seriously hamper language development. What factors internal to the organism might be responsible for the difference between the treatment of triggers in L1 and L2 acquisition? Newport (1990) suggests that there may be a neurobiological factor relevant for the critical period which results in bound morphemes being processed by second language learners. We suspect, however, that ultimately the distinction between bound and free morphemes as triggers may be derived from phonology - free morphemes typically constituate at least a phonological foot, while bound morphemes typically involve units smaller than a foot.