Chapter 4 outlines key properties of language development and explores the question whether the acquisition of an additional or foreign language is fundamentally different compared to the acquisition of a primary language in infancy. Drawing on key findings from studies of second language acquisition involving a range of language pairs, it is shown that some properties of an additional language are acquired naturally despite apparent difficulties caused by input and transfer from previous language knowledge. However, some properties of grammar and language use remain persistently difficult, even despite input and teaching of the properties. We conclude with the claim that the knowledge of Virtual Grammar and its grammatical concepts provides a way for teachers to understand the learnability issues faced by their language learners so that they can then make use of appropriate approaches to address these issues.