This chapter provides a brief introduction to the Chinese language, including its main properties, such as its writing system, the use of pinyin, tones, word order, classifiers, word-formation, temporal and aspectual marking, sentence-final particles, topicalisation, passives, and relative clauses. It explains the book’s focus on the mental representations of various aspects of learners’ L2 Chinese grammars and scrutinises these representations based on data from a range of empirical studies. It specifies the featural approach adopted and explains that all morphosyntactic variations, both within and across languages, are perceived to be encoded in the lexicon in terms of features. It is argued that this decompositional approach to lexical items is essential to any account of language acquisition and can help to advance our knowledge of the underlying mechanisms in interlanguage grammars. Influential theories on L2 acquisition are reviewed, and with examples from both L2 English and L2 Chinese, cues in the target language input are categorised into macro-, micro-, and nano-cues as well as absence of cues with regard to their salience, detectability, and availability in the input. These are assumed to form a scalar continuum with no clear boundaries between the categories.