Aims: In this introduction we situate the seven articles in this special issue in terms of the connections between their themes and their individual contributions to the field of third language acquisition: new theoretical models, innovative methodologies, an epistemological commentary and new perspectives related to multilingual processing and cognitive function. Approach:We discuss important and often overlooked differences between bi-and multilingualism in the context of L2 vs. L3/Ln acquisition. We also provide a brief historical overview of the relatively young field of L3A and outline the three current models of linguistic transfer in L3 morphosyntax. Finally, we approach the issues of methodology, psychological complexity and cognitive implications that are discussed in some of these papers. Conclusions:The diversity of topics in these articles endows the issue with a broad approach to the field of L3A, while individual articles offer a narrow focus on specific theoretical and methodological issues.Significance: This special issue provides an accurate portrayal of the current interest in, and rapid expansion of, multilingualism within linguistic and psycholinguistic approaches.2

We examine the acquisition of progressivity in the German simple present tense (GPT) and related constructions by English natives of L2 German, under the guise of the Feature Reassembly Hypothesis. German lacks an explicit gerundial form; the AM+infinitive (am) and BEIM+infinitive (beim) constructions express progressivity, along with the GPT. Although semantically the am and beim constructions map to the English copula+gerund, their unique syntax creates grammaticality differences. We employ three experiments testing (1) the acquisition of the aspectual properties of the GPT, and knowledge of the (2) semantic distribution of, and (3) syntax of am and beim. Data from a control group (n = 25) and adult L2s (n = 20) suggest adult L2 feature reassembly is possible, albeit complicated.