Chapter 4 explores potential advantages of multilingual upbringing in relation to further language development. It argues that any such advantages play out more forcefully and are easier to identify in regard to general proficiencies rather than specific grammatical phenomena. Chapter 4 further argues that type of bilingualism, as operationalized in terms of language dominance, and the type of language knowledge investigated play a pivotal role for understanding language learning processes against a multilingual substrate. Cross-linguistic influence, as observable in different grammatical domains and the lexicon, can aid additional language acquisition by means of cumulative enhancement, but certain constellations also produce inhibitory effects. The assumption of universally facilitating effects has turned out to be overly optimistic and needs to be tempered. Much of the discussion in Chapter 4 is based on the acquisition of English as an additional language in bilingual heritage contexts (third language acquisition). Besides general CEFR-proficiencies, the case studies concern determiners, subject–verb agreement, tense and aspect, word order, and lexical cross-linguistic influence.