A plethora of previous research has shown early beneficial effects of bilingualism and second language acquisition (SLA) on cognitive control (CC) (e.g. Crivello et al., 2016), working memory (WM), and intelligence (IQ) (e.g. Woumans et al., 2016). However, the reliability and validity of these so-called bilingual advantage effects is also disputed (Paap et al., 2015), and the reported between-group comparisons imply the risk for relevant group differences. This necessitates a longitudinal design, using much larger samples. Therefore, we will investigate the effects of bilingualism in a longitudinal design, evaluating cognitive development within groups of initially equivalent participants. In a first study, we will annually test three different groups (bilinguals, monolinguals, and second- language learners) of - initially - four-year-old participants, in order to investigate the effect of SLA on CC, WM, and IQ, but also on first language/vocabulary acquisition throughout three years that are critical for cognitive development. In a second study, we will also investigate whether such differences remain visible in post-critical age groups of bilinguals, monolinguals, and second-language learners of seven to nine years old, also matched on relevant background variables. This allows to investigate both the development of cognitive bilingual advantages at a critical age and the longevity of such effects at a post-critical age in a four-year project.