This books aims to open up new perspectives in the study of language proficiency by bringing together current research from different fields in psychology and linguistics. All contributions start out from empirical studies, which are then related to applications in language assessment. The book also serves as a survey of recent developments in psycholinguistic research in the Netherlands. The book starts out with a thorough introduction of international literature on models of language proficiency, language development and its assessment. Section 1 deals with first language proficiency and addresses such problems as grammar in early child language, grammatical proficiency and its (in)variance across a range of ages, reading abilities, and writing skills. Section 2 focuses on multilingual proficiency and deals with test bias in relation to the background of the second language learner, bilingual proficiency in ethnic minority children, the development of the second language learner lexicon, communicative competence of school-age children in the context of second language learning, the assessment of foreign language attrition and the dimensionality in oral foreign language proficiency.