We have witnessed a growing number of investigations into the acquisition process in a multilingual context, which has become recognized as an independent field, quantitatively and qualitatively different from second language acquisition. Scholars have started to differentiate between learners/speakers on the basis of the complexity of their linguistic background, with the numer of known languages being an additional variable. A growing body of studies into the acquisition of third language phonology demonstrates an inherent complexity of the field reflected, among others, in multidirectional dynamic cross-linguistic influence. As shown, multilingual learners have at their disposal a broadened phonetic repertoire, a raised level of metalinguistic awareness and enhanced perceptual sensitivity, which may facilitate the learning of subsequent phonological systems. Thus, this chapter aims to compare bilingual and trilingual phonetics and phonology by providing an overview of recent research into both subdomains, identifying their common features and, importantly, their points of departure for L3 phonology, with a view to providing new insights into the acquisition of speech.