Countering traditional monolingual ideologies that associate multilingual development with various deficiencies, more recent research probes into potential advantages of this developmental experience. The discussion about putative multilingual advantages remains highly controversial and also emotional, as it affects far-reaching policy decisions. The chapter attempts to provide an objective state-of-the-art report, discussing research findings on executive function (control), cognitive reserve, cognitive development, educational attainment, and metalinguistic awareness. It furthermore tries to identify the boundary conditions that help to explain why some studies report positive results while others do not. Current research suggests that the characteristics of the speaker groups sampled, especially in terms of their type of bilingualism and multilingualism (balanced, unbalanced, heritage speakers, etc.), offer important clues for a better understanding of this domain. Moreover, the social prestige associated with the languages studied appears to influence the results considerably. Language users are not deterministic machines, but react to social pressure in intelligent ways.