In this chapter, we reinforce the book’s aim to shed light on changes inflicted on language, cognition, and the brain rather than to focus on advantages and disadvantages of being bilingual. To obtain a more realistic picture of bilingualism, its assets (i.e., what is easier), and its difficulties (i.e., what is taxing and leads to high consumption of mental resources), we have drawn on research from various disciplines. We conclude the book by identifying complexity as the major issue for research on bilingualism. The complexity problem is fundamental to definitions of bilingualism and the characterization of bilingual participants in empirical studies, leading to discussions about its assessment as a dichotomous or continuous variable. Considering bilingualism as an experience and how such experience impacts overall language development, cognition, and the brain at different levels are related to usage-based approaches of examining bilingualism as well as a concern regarding confounding and moderating variables. The shift for designing research in the field of bilingualism seems to necessarily be more interdisciplinary in nature than in the past.