Much research on bilingual first language acquisition has stressed the role of the dominant or preferred language when the two languages have some influence on one another. The present paper tries to look at transfer or interference from the perspective of the input the child is exposed to. Transfer will be argued to occur in those domains of the grammar where the language learner is confronted with ambiguous input. The bilingual child may, as a relief strategy, use parts of the analysis of one language in order to cope with ambiguous properties of the other. Ambiguity of input is crucial and will be evaluated through a comparison with monolingual language acquisition: if monolingual children have problems with the language material in question, it may be suggested that the input contains evidence for more than only one grammatical analysis. A quantitative difference between monolingual and bilingual language acquisition will be interpreted as evidence in favor of cross-linguistic influence in bilingual language development. The paper reviews longitudinal studies on the acquisition of word order in German subordinate clauses.