From the beginning of this century fascinating work has been carried out on the simultaneous acquisition of two languages from birth in the form of diary studies for example, that by Ronjat (1913). Many aspects of monolingual language acquisition have been studied in the same way, whereby researchers generally observed their own children, for example, Stern and Stern (1928). The modern research strategy is to audiotape or videotape the speech of young children at fixed intervals. Of course, a quantitative analysis can be carried out for both diary studies and more recent types of longitudinal study. However, researchers such as Ronjat (1913) do not provide us with exact quantitative results. Notwithstanding, these studies deserve to be taken seriously and I think it is possible to interpret some of the observations they contain as good and “reliable” evidence and compare these with results from modern longitudinal studies, where exact quantification is an important issue. I therefore think that the quantitative results of my review of existing studies on the acquisition of German subordinate clauses in bilingual children can be taken seriously, although I agree with François Grosjean and Elizabeth Lanza, who object that the quantitative differences between monolingual and bilingual children's errors are perhaps artefacts of the setting in which the children have been recorded. That I considered diary studies as well disappointed one commentator, Annick De Houwer, who would have preferred a comparison of modern studies only.