This book presents a comprehensive, state-of-the-art treatment of the acquisition of Indo- and Non-Indo-European languages in various contexts, such as L1, L2, L3/Ln, bi/multilingual, heritage languages, pathology as well as language impairment, and sign language acquisition. The book explores a broad mix of methodologies and issues in contemporary research. The text presents original research from several different perspectives, and provides a basis for dialogue between researchers working on diverse projects with the aim of furthering our understanding of how languages are acquired.  The book proposes and refines new theoretical constructs, e.g. regarding the complexity of linguistic features as a relevant factor forming children’s, adults’ and bilingual individuals’ acquisition of morphological, syntactic, discursive, pragmatic, lexical and phonological structures.  It appeals to students, researchers, and professionals in the field.