In this chapter, we consider different definitions of bilingualism, underscoring the reality that there are various aspects which should be taken into account when investigating bilingualism, particularly when designing studies and choosing participants. Bilingualism is a complex construct and should be viewed on a continuum. Crucially, many key details about bilinguals’ backgrounds need to be reported in studies to make results comparable and clearly linkable to the specific study sample. Relative proficiency level seems to be the most influential factor, but it is by no means the only factor relevant for studying bilingualism. Rather, individual differences and their variability, dynamically related dimensions and their interaction over time, speech environment and their changes, language use habits, socioeconomic background, and so on have been reported to influence language processing and even brain function to some extent.

More than half of the world’s population is bilingual. However, ‘bilingual’ can mean different things with respect to individuals’ linguistic competence, ranging from two native competences to basic communicative abilities. Uncertainties about the nature of the attainable linguistic knowledge in both languages leads to negative attitudes towards child bilingualism. Parents might therefore decide against bilingualism. This can contribute to the decline of minority languages. An example is Occitan. It is therefore crucial to disentangle facts and myths concerning risks and benefits of child bilingualism. Since these are well-studied issues, the search for facts can draw on resarch results that provide solid knowledge about the linguistic knowledge and skills of young bilinguals. The focus lies on the acquisition of grammatical knowledge and the ability to use it in communication. The goal is to provide information allowing parents to make informed choices when deciding on whether to raise children bilingually. All recommendations are based on research findings, some of which are summarized in the following chapters. These are organized in such a way as to allow readers to focus on the aspects that are most relevant to them.