There is growing recognition that multilingualism is a norm rather than an exception and that it constitutes a default state of human linguistic competence. This phenomenon has become particularly relevant due to widespread, yet variously motivated, population movements as well as job-related mobility and the introduction of multiple foreign languages into school curricula. It has resulted in the emergence of a complex linguistic landscape and raised many questions concerning language development, multilingual use, and cross-linguistic interactions. Consequently, there has been a growing need for investigations into the process of multiple language acquisition since, as some scholars argue, the study of third language acquisition (TLA) can offer new insights into the process of language learning that research on the first or the second language alone cannot provide (e.g. Flynn, Foley, & Vinnitskaya, 2004). TLA research findings generated so far generally demonstrate complex patterns of transfer between native and nonnative languages; yet, they are insufficient to provide a full account of third language (L3) phonological modelling and the factors conditioning cross-linguistic influence in L3 speech. Therefore, the main objective of this special issue is to shed additional light on the advancements in research on third language phonological acquisition. Further, it aims to contribute to the development of this field as well as to language acquisition research in a broad sense. This article serves as a state of the science. The authors begin with a brief overview of studies in the field to establish major findings and limitations, and segue into different methodological issues that this work has yielded. They then outline the major research questions that drive the field of study, and discuss the advances that each of the contributions makes towards answering these questions. Finally, they complement the theoretical discussion with the methodological advances made by these studies and consider some future directions for the area.