This chapter conceptualizes and discusses two subtypes of multilingualism, namely, (a) third language acquisition in learners who have prior experience in acquiring one or more non-native languages, and (b) subsequent language acquisition in learners who are bilingual from an early age. The chapter also discusses the roles played by age (both biological age and age of onset) and proficiency in multilingual acquisition. As regards age, the discussion focuses primarily on two aspects, namely, differential effects in instructed and naturalistic contexts, and the apparent superiority of older learners and late starters against younger learners and early starters. It is stressed that further research is necessary in order to identify and single out the particularities of L3 acquisition for learners with prior experience in the concurrent or consecutive learning of one or more non-native languages. Likewise, the chapter highlights the need to obtain a deeper understanding of how age-related differences in the level of linguistic entrenchment in multilinguals constrain L3 learning after exposure to additional languages. With respect to the proficiency factor, it is argued that it is important to consider proficiency thresholds and to tease apart the distinctive effects of proficiency in the target language (the L3) and in the background languages when exploring linguistic development among multilinguals. These distinctive effects are also relevant for understanding crosslinguistic influence in L3 acquisition, above all in determining the potential source languages and the direction of this influence.