Children seem to acquire their mother tongue usually without special effort, though this cannot be said of adults acquiring a new language. More particularly, adult L2 learners have difficulties in the realisation of (ad)nominal and verbal inflection. The question as to why adult L2 learners of Dutch have these difficulties is the topic of this book. Traditionally, morphosyntactic explanations have been given to clarify the difficulties L2 learners have in acquiring L2 inflectional morphology. Here , it is claimed that morphosyntactic accounts alone cannot explain all problems L2 learners have in acquiring inflectional morphology. Phonetic-phonological constraints should be taken into account as well. To investigate this claim, a corpus study was done and two processing experiments were conducted to test lower-educated Turkish, Moroccan Arabic and Mandarin Chinese learners of Dutch on their command of Dutch inflectional morphology. The use of both production and perception data, the systematic incorporation of different L1 backgrounds and L2 proficiency levels and the large number of participants included make the design of this study truly unique. This study is of interest to scholars working in the field of L2 acquisition, inflectional morphology, phonology, and morphosyntax as well as to educators working with L2 learners. It shows that the acquisition of inflectional morphology in a second language comprises more than applying a grammatical rule. Accurate perception and production of the phonemes serving as morphemes are of crucial importance as well.